[macOS] Tree widget NullPointerException in checkData() - cannot read field "cached" because "item" is null

[qforce85]

Describe the bug

SWT Tree widget throws NullPointerException during normal operation on macOS. The error occurs in Tree.checkData() method when trying to access the "cached" field of a null "item" object. This happens during tree rendering/display operations and results in cascading failures with multiple suppressed exceptions.

Stack trace:

java.lang.NullPointerException: Cannot read field "cached" because "item" is null
	at org.eclipse.swt.widgets.Tree.checkData(Tree.java:328)
	at org.eclipse.swt.widgets.Tree.outlineView_objectValueForTableColumn_byItem(Tree.java:2238)
	at org.eclipse.swt.widgets.Display.windowProc(Display.java:6764)
	at org.eclipse.swt.internal.cocoa.OS.objc_msgSendSuper(Native Method)
	at org.eclipse.swt.widgets.Display.applicationSendEvent(Display.java:5705)
	at org.eclipse.swt.widgets.Display.applicationProc(Display.java:5845)
	at org.eclipse.swt.internal.cocoa.OS.objc_msgSend(Native Method)
	at org.eclipse.swt.internal.cocoa.NSApplication.sendEvent(NSApplication.java:117)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:4000)
	at com.docfetcherpro.util.UtilSwt$.mainLoopHandled(UtilSwt.scala:146)
	at com.docfetcherpro.MainDesktop$.main(MainDesktop.scala:238)
	at com.docfetcherpro.MainDesktop.main(MainDesktop.scala)
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "cached" because "item" is null
		... 12 more
	Suppressed: java.lang.NullPointerException: Cannot read field "image" because "item" is null
		at org.eclipse.swt.widgets.Tree.outlineView_willDisplayCell_forTableColumn_item(Tree.java:2316)
		at org.eclipse.swt.widgets.Display.windowProc(Display.java:6814)
		... 9 more

To Reproduce

According to the user who reported this crash, the application was "open and idle in the background for days" before they interacted with the UI and triggered the crash. I have not seen this specific crash reported before across my userbase. This suggests the issue might be difficult or impossible to reproduce reliably.

Expected behavior

Tree widget should display normally without throwing NullPointerException.

Environment:

1. Select the platform(s) on which the behavior is seen:

• All OS
• Windows
• Linux
• macOS

2. Additional OS info (e.g. OS version, Linux Desktop, etc)
   macOS 14.7 on x86_64

3. JRE/JDK version
   OpenJDK Runtime Environment 17.0.10+7

Version since

SWT 4.34

Workaround (or) Additional context

No known workaround at this time.

[merks]

You should consider providing a snippet that reproduces the problem. If no one else can reproduce it, it’s highly unlikely to get attention. Testing 4.36 would also be a good idea. Setting up a development environment for contribution is super easy

https://github.com/eclipse-platform/eclipse.platform.swt?tab=readme-ov-file#contributing-to-swt

Doing so will help highlight what someone else needs to do to help you fix the problem.

[qforce85]

I can't reproduce the crash, and the user who reported it to me most likely can't reproduce it either. However, I asked Claude AI (specifically Claude Sonnet 4 with Thinking) to take a look at the stacktrace and the relevant source code. It suggests adding an NPE check at a specific location, and similar checks at other locations. Here's Claude's full analysis:

---

After examining the SWT Tree source code, I've identified the exact root cause of this NPE:

Root Cause Analysis

The crash occurs in this call chain:

1. Native Cocoa NSOutlineView calls outlineView_objectValueForTableColumn_byItem()
2. This method calls TreeItem item = (TreeItem) display.getWidget(itemID)
3. display.getWidget() returns null (likely because the widget was disposed or the ID is stale)
4. The method then calls checkData(item) where item is null
5. checkData() immediately tries to access item.cached on the first line, causing the NPE:

boolean checkData (TreeItem item) {
    if (item.cached) return true;  // NPE here when item is null
    // ...
}

Why This Happens After Long Runtime

This explains the "open and idle in the background for days" timing:

• TreeItems get created and disposed throughout the application's lifetime
• The native NSOutlineView retains references to item IDs that no longer exist
• Some trigger (system wake, view refresh, memory pressure, user interaction) causes the native view to attempt rendering
• The native view tries to access TreeItems using stale IDs
• SWT's widget registry no longer has those items → getWidget() returns null → crash

Suggested Fix

Add a null check in outlineView_objectValueForTableColumn_byItem():

@Override
long outlineView_objectValueForTableColumn_byItem (long id, long sel, long outlineView, long tableColumn, long itemID) {
    TreeItem item = (TreeItem) display.getWidget (itemID);
    if (item == null) {
        // Handle stale itemID - return appropriate default value
        return 0; // or NSString.string().id for empty string
    }
    checkData (item);
    // ... rest of method
}

This same null check pattern should probably be applied to other native callback methods that retrieve TreeItems by ID, such as:

• outlineView_willDisplayCell_forTableColumn_item()
• outlineView_isItemExpandable()
• outlineView_numberOfChildrenOfItem()

Why This Bug Is Rare

This bug requires a specific combination of conditions:

1. Long-running application (hours/days)
2. TreeItems being created and disposed over time
3. Native view retaining stale references
4. A trigger that causes the native view to refresh/render
5. Timing where the widget registry cleanup has occurred but native references haven't

This explains why it's not commonly reported and is difficult to reproduce reliably.

[qforce85]

I threw all the relevant classes (including the SWT Cocoa internals) at Gemini 2.5 Pro. It agrees with Claude and identified the methods in Tree.java that need NPE or disposal checks. Here are the updated methods (based on SWT 4.34), with markers "ADDED" and "MODIFIED" to point out what was added or modified.

boolean checkData (TreeItem item) {
	/* ADDED */
	if (item == null) return false;
	
	if (item.cached) return true;
	if ((style & SWT.VIRTUAL) != 0) {
		item.cached = true;
		Event event = new Event ();
		TreeItem parentItem = item.getParentItem ();
		event.item = item;
		event.index = parentItem == null ? indexOf (item) : parentItem.indexOf (item);
		ignoreRedraw = true;
		sendEvent (SWT.SetData, event);
		//widget could be disposed at this point
		ignoreRedraw = false;
		if (isDisposed () || item.isDisposed ()) return false;
		if (!setScrollWidth (item)) item.redraw (-1);
	}
	return true;
}

@Override
long outlineView_objectValueForTableColumn_byItem (long id, long sel, long outlineView, long tableColumn, long itemID) {
	TreeItem item = (TreeItem) display.getWidget (itemID);
	
	/* ADDED */
	if (item == null || item.isDisposed()) {
		return NSString.string().id;
	}

	/* MODIFIED */
	// checkData (item);
	if (!checkData (item)) {
		return NSString.string().id;
	}
	
	if (checkColumn != null && tableColumn == checkColumn.id) {
		NSNumber value;
		if (item.checked && item.grayed) {
			value = NSNumber.numberWithInt (OS.NSMixedState);
		} else {
			value = NSNumber.numberWithInt (item.checked ? OS.NSOnState : OS.NSOffState);
		}
		return value.id;
	}
	for (int i=0; i<columnCount; i++) {
		if (columns [i].nsColumn.id == tableColumn) {
			return item.createString (i).id;
		}
	}
	return item.createString (0).id;
}

@Override
boolean outlineView_isItemExpandable (long id, long sel, long outlineView, long itemID) {
	if (itemID == 0) return true;
	/* MODIFIED */
	// return ((TreeItem) display.getWidget (item)).itemCount != 0;
	TreeItem item = (TreeItem) display.getWidget (itemID);
	if (item == null || item.isDisposed()) return false;
	return item.itemCount != 0;
}

@Override
long outlineView_numberOfChildrenOfItem (long id, long sel, long outlineView, long itemID) {
	if (itemID == 0) return itemCount;
	/* MODIFIED */
	// return ((TreeItem) display.getWidget (item)).itemCount;
	TreeItem item = (TreeItem) display.getWidget (itemID);
	if (item == null || item.isDisposed()) return 0;
	return item.itemCount;
}

@Override
boolean outlineView_shouldTrackCell_forTableColumn_item(long id, long sel, long table, long cell, long tableColumn, long itemID) {
	if ((style & SWT.CHECK) != 0) {
		/* MODIFIED */
		// if (new NSCell(cell).isKindOfClass(OS.class_NSButtonCell)) return true;
		if (new NSCell(cell).isKindOfClass(OS.class_NSButtonCell)) {
			TreeItem item = (TreeItem) display.getWidget(itemID);
			return item != null && !item.isDisposed();
		}
	}
	NSOutlineView widget = (NSOutlineView)view;
	
	/* ADDED */
	id nsItem = new id(itemID);
	TreeItem item = (TreeItem) display.getWidget(nsItem.id);
	if (item == null || item.isDisposed()) return false;

	long rowIndex = widget.rowForItem(nsItem);
	return widget.isRowSelected(rowIndex);
}

@Override
void outlineView_willDisplayCell_forTableColumn_item (long id, long sel, long outlineView, long cell, long tableColumn, long itemID) {
	if (checkColumn != null && tableColumn == checkColumn.id) return;
	TreeItem item = (TreeItem) display.getWidget(itemID);
	
	/* ADDED */
	// If item is invalid, configure the cell with default/empty values to avoid
	// displaying stale data or crashing.
	if (item == null || item.isDisposed()) {
		if (cell == 0) {
			return;
		}
		NSCell nsCell = new NSCell(cell);
		if (!nsCell.isKindOfClass(OS.class_NSTextFieldCell)) {
			return;
		}
		NSTextFieldCell textCell = new NSTextFieldCell(cell);
		textCell.setObjectValue(NSString.string());
		textCell.setImage(null);

		// Reset font to Tree's current font or system default
		Font currentTreeFont = getFont();
		textCell.setFont(currentTreeFont != null && currentTreeFont.handle != null ? currentTreeFont.handle : (display.outlineViewFont != null ? display.outlineViewFont : NSFont.systemFontOfSize(NSFont.systemFontSize())));

		// Reset text color to Tree's current foreground or system default
		Color currentTreeFgColor = getForeground();
		double[] rgba = currentTreeFgColor != null && currentTreeFgColor.handle != null ? currentTreeFgColor.handle : display.getWidgetColorRGB(SWT.COLOR_LIST_FOREGROUND);
		NSColor nsColor = NSColor.colorWithDeviceRed(rgba[0], rgba[1], rgba[2], rgba[3]);
		textCell.setTextColor(nsColor);

		textCell.setAlignment(OS.NSTextAlignmentLeft);
		textCell.setBaseWritingDirection( (style & SWT.RIGHT_TO_LEFT) != 0 ? OS.NSWritingDirectionRightToLeft : OS.NSWritingDirectionLeftToRight );
		return;
	}

	int index = 0;
	for (int i=0; i<columnCount; i++) {
		if (columns [i].nsColumn.id == tableColumn) {
			index = i;
			break;
		}
	}
	NSTextFieldCell textCell = new NSTextFieldCell (cell);
	OS.object_setInstanceVariable(cell, Display.SWT_ROW, itemID);
	OS.object_setInstanceVariable(cell, Display.SWT_COLUMN, tableColumn);
	Image image = index == 0 ? item.image : (item.images == null ? null : item.images [index]);
	textCell.setImage (image != null ? image.handle : null);
	NSColor color;
	if (textCell.isEnabled()) {
		if (textCell.isHighlighted ()) {
			color = NSColor.selectedControlTextColor();
		} else {
			Color foreground = item.cellForeground != null ? item.cellForeground [index] : null;
			if (foreground == null) foreground = item.foreground;
			if (foreground == null) foreground = getForegroundColor ();
			color = NSColor.colorWithDeviceRed (foreground.handle [0], foreground.handle [1], foreground.handle [2], 1);
		}
	} else {
		color = NSColor.disabledControlTextColor();
	}
	int direction = (style & SWT.RIGHT_TO_LEFT) != 0 ? OS.NSWritingDirectionRightToLeft : OS.NSWritingDirectionLeftToRight;
	int alignment = OS.NSTextAlignmentLeft;
	if (columnCount > 0) {
		int style = columns [index].style;
		if ((style & SWT.CENTER) != 0) {
			alignment = OS.NSTextAlignmentCenter;
		} else if ((style & SWT.RIGHT) != 0) {
			alignment = OS.NSTextAlignmentRight;
		}
	}
	Font font = item.cellFont != null ? item.cellFont [index] : null;
	if (font == null) font = item.font;
	if (font == null) font = this.font;
	if (font == null) font = defaultFont ();
	if (font.extraTraits != 0) {
		NSMutableDictionary dict = ((NSMutableDictionary)new NSMutableDictionary().alloc()).initWithCapacity(5);
		dict.setObject (color, OS.NSForegroundColorAttributeName);
		dict.setObject (font.handle, OS.NSFontAttributeName);
		addTraits(dict, font);
		NSMutableParagraphStyle paragraphStyle = (NSMutableParagraphStyle)new NSMutableParagraphStyle ().alloc ().init ();
		paragraphStyle.setLineBreakMode (OS.NSLineBreakByClipping);
		paragraphStyle.setAlignment (alignment);
		paragraphStyle.setBaseWritingDirection(direction);
		dict.setObject (paragraphStyle, OS.NSParagraphStyleAttributeName);
		paragraphStyle.release ();
		NSAttributedString attribStr = ((NSAttributedString) new NSAttributedString ().alloc ()).initWithString (textCell.title(), dict);
		textCell.setAttributedStringValue(attribStr);
		attribStr.release();
		dict.release();
	} else {
		textCell.setFont(font.handle);
		textCell.setTextColor(color);
		textCell.setAlignment (alignment);
		textCell.setBaseWritingDirection(direction);
	}
}

@Override
void outlineView_setObjectValue_forTableColumn_byItem (long id, long sel, long outlineView, long object, long tableColumn, long itemID) {
	if (checkColumn != null && tableColumn == checkColumn.id)  {
		TreeItem item = (TreeItem) display.getWidget (itemID);
		
		/* ADDED */
		if (item == null || item.isDisposed()) return;
		
		item.checked = !item.checked;
		Event event = new Event ();
		event.detail = SWT.CHECK;
		event.item = item;
		sendSelectionEvent (SWT.Selection, event, false);
		item.redraw (-1);
	}
}

[qforce85]

Verbal summary of the changes, written by Gemini 2.5 Pro:

---

The primary goal of the changes is to prevent NullPointerExceptions (NPEs) that can occur in various NSOutlineView delegate/datasource callback methods within the Tree.java class. These NPEs arise when the native macOS framework requests data for or interacts with a TreeItem that SWT no longer considers valid (i.e., it's null after a lookup from a native ID, or it has been disposed). This situation can be triggered, for instance, if an application remains idle for a long time, potentially leading to desynchronization between the SWT widget state and the native view's expectations.

The fix involves the following key strategies across several callback methods:

1. Robust Item Retrieval and Validation:

   • In methods like outlineView_objectValueForTableColumn_byItem, outlineView_isItemExpandable, outlineView_numberOfChildrenOfItem, outlineView_setObjectValue_forTableColumn_byItem, and outlineView_shouldTrackCell_forTableColumn_item, the TreeItem is retrieved from the native itemID using display.getWidget().
   • Immediately after retrieval, a check if (item == null || item.isDisposed()) is performed.
   • If the item is invalid, the method either returns a safe default value (e.g., an empty NSString ID, false, or 0) or performs a no-op, thus avoiding any attempt to access fields or methods of a null or disposed TreeItem.

2. Safe Cell Configuration for Invalid Items in outlineView_willDisplayCell_forTableColumn_item:

   • This method, responsible for preparing a cell before display, also checks if the corresponding TreeItem is null or disposed.
   • If the item is invalid, and the cell is an NSTextFieldCell (the common case), the cell's content is cleared (text set to empty, image to null).
   • Crucially, the cell's visual attributes (font, text color, alignment, writing direction) are reset to match the Tree's current default styling or system defaults. This prevents visual glitches where a reused cell might retain styling from a previously valid but differently styled item.

3. Safer checkData Method:

   • The checkData(TreeItem item) method itself is made more robust by adding a null check for the item parameter at its very beginning. If item is null, it immediately returns false. This complements the checks in the calling methods.
   • In outlineView_objectValueForTableColumn_byItem, an additional check is made after calling checkData(item) because checkData can, in virtual trees, trigger SWT.SetData listeners which might dispose the item.

In essence, the changes ensure that SWT's Tree widget gracefully handles requests from the native NSOutlineView for items that are no longer valid from SWT's perspective. This increases the stability of the Tree widget, especially in scenarios where the UI might have been idle or subject to system events that could affect synchronization. The cell styling reset also improves visual consistency.