Fix incorrect double to int cast, and guard against optional parent (singerdmx#239)

Author: hyouuu

lib/src/models/documents/nodes/node.dart

@@ -53,6 +53,9 @@ abstract class Node extends LinkedListEntry<Node> {
 
   /// Offset in characters of this node in the document.
   int get documentOffset {
+    if (parent == null) {
+      return offset;
+    }
     final parentOffset = (parent is! Root) ? parent!.documentOffset : 0;
     return parentOffset + offset;
   }

lib/src/widgets/text_line.dart

@@ -625,11 +625,11 @@ class RenderEditableTextLine extends RenderEditableBox {
     final verticalPadding = _resolvedPadding!.top + _resolvedPadding!.bottom;
     final leadingWidth = _leading == null
         ? 0
-        : _leading!.getMinIntrinsicWidth(height - verticalPadding) as int;
+        : _leading!.getMinIntrinsicWidth(height - verticalPadding).floor();
     final bodyWidth = _body == null
         ? 0
         : _body!.getMinIntrinsicWidth(math.max(0, height - verticalPadding))
-            as int;
+            .floor();
     return horizontalPadding + leadingWidth + bodyWidth;
   }
 
@@ -640,11 +640,11 @@ class RenderEditableTextLine extends RenderEditableBox {
     final verticalPadding = _resolvedPadding!.top + _resolvedPadding!.bottom;
     final leadingWidth = _leading == null
         ? 0
-        : _leading!.getMaxIntrinsicWidth(height - verticalPadding) as int;
+        : _leading!.getMaxIntrinsicWidth(height - verticalPadding).ceil();
     final bodyWidth = _body == null
         ? 0
         : _body!.getMaxIntrinsicWidth(math.max(0, height - verticalPadding))
-            as int;
+            .ceil();
     return horizontalPadding + leadingWidth + bodyWidth;
   }