fixes crash when sets mouseTrackingMode before view is loaded (flutter#26576)

Author: chunhtai

shell/platform/darwin/macos/framework/Source/FlutterViewController.mm

@@ -396,7 +396,11 @@ - (void)listenForMetaModifiedKeyUpEvents {
 }
 
 - (void)configureTrackingArea {
-  NSAssert(self.viewLoaded, @"View must be loaded before setting tracking area");
+  if (!self.viewLoaded) {
+    // The viewDidLoad will call configureTrackingArea again when
+    // the view is actually loaded.
+    return;
+  }
   if (_mouseTrackingMode != FlutterMouseTrackingModeNone && self.flutterView) {
     NSTrackingAreaOptions options = NSTrackingMouseEnteredAndExited | NSTrackingMouseMoved |
                                     NSTrackingInVisibleRect | NSTrackingEnabledDuringMouseDrag;

shell/platform/darwin/macos/framework/Source/FlutterViewControllerTest.mm

@@ -140,6 +140,16 @@ + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
   EXPECT_EQ([window firstResponder], viewController.flutterView);
 }
 
+TEST(FlutterViewController, CanSetMouseTrackingModeBeforeViewLoaded) {
+  NSString* fixtures = @(testing::GetFixturesPath());
+  FlutterDartProject* project = [[FlutterDartProject alloc]
+      initWithAssetsPath:fixtures
+             ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithProject:project];
+  viewController.mouseTrackingMode = FlutterMouseTrackingModeInActiveApp;
+  ASSERT_EQ(viewController.mouseTrackingMode, FlutterMouseTrackingModeInActiveApp);
+}
+
 TEST(FlutterViewControllerTest, TestKeyEventsAreSentToFramework) {
   ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testKeyEventsAreSentToFramework]);
 }