Import a container, let's say a VBox, then add a TreeView to the VBox.
View sample data for the TreeView.
Try to reparent the TreeView to the root AnchorPane. You will get the exception below.
At this point, the tool is not in a good shape (cannot remove the TreeView...).

Note that if you reparent to a node that is not root, or if you reparent in the hierarchy, you will not get the exception.

Glass detected outstanding Java exception at -[GlassViewDelegate sendJavaMouseEvent:]:src/com/sun/mat/ui/GlassViewDelegate.m:541
Exception in thread "AWT-AppKit" java.lang.RuntimeException: java.lang.AssertionError: 23 != 24
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:251)
at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
at com.sun.glass.ui.View.notifyMouse(View.java:922)
Caused by: java.lang.AssertionError: 23 != 24
at com.sun.javafx.css.StyleHelper.getCacheEntry(StyleHelper.java:379)
at com.sun.javafx.css.StyleHelper.transitionToState(StyleHelper.java:749)
at javafx.scene.Node.impl_processCSS(Node.java:7419)
at javafx.scene.Parent.impl_processCSS(Parent.java:1146)
at javafx.scene.control.Control.impl_processCSS(Control.java:1102)
at com.sun.javafx.scene.control.skin.VirtualFlow.setCellIndex(VirtualFlow.java:1598)
at com.sun.javafx.scene.control.skin.VirtualFlow.getCell(VirtualFlow.java:1500)
at com.sun.javafx.scene.control.skin.VirtualFlow.getCellLength(VirtualFlow.java:1523)
at com.sun.javafx.scene.control.skin.VirtualFlow$3.call(VirtualFlow.java:478)
at com.sun.javafx.scene.control.skin.VirtualFlow$3.call(VirtualFlow.java:476)
at com.sun.javafx.scene.control.skin.PositionMapper.computeViewportOffset(PositionMapper.java:143)
at com.sun.javafx.scene.control.skin.VirtualFlow.layoutChildren(VirtualFlow.java:1001)
at com.sun.javafx.scene.control.skin.VirtualFlow.setCellCount(VirtualFlow.java:206)
at com.sun.javafx.scene.control.skin.TreeViewSkin.updateItemCount(TreeViewSkin.java:225)
at com.sun.javafx.scene.control.skin.TreeViewSkin.handleControlPropertyChanged(TreeViewSkin.java:135)
at com.sun.javafx.scene.control.skin.SkinBase$3.changed(SkinBase.java:282)
at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:107)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:196)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100)
at javafx.beans.property.IntegerPropertyBase.fireValueChangedEvent(IntegerPropertyBase.java:123)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:130)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:163)
at javafx.scene.control.TreeView.setTreeItemCount(TreeView.java:515)
at javafx.scene.control.TreeView.updateTreeItemCount(TreeView.java:876)
at javafx.scene.control.TreeView.updateRootExpanded(TreeView.java:928)
at javafx.scene.control.TreeView.access$400(TreeView.java:118)
at javafx.scene.control.TreeView$2.invalidated(TreeView.java:358)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:129)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:163)
at javafx.scene.control.TreeView.setRoot(TreeView.java:378)
at com.oracle.javafx.authoring.runtime.SampleData$SampleDataVisitor.visitTree(SampleData.java:409)
at com.oracle.javafx.authoring.runtime.SampleData$SampleDataVisitor.visit(SampleData.java:473)
at com.oracle.javafx.authoring.runtime.SampleData$SampleDataVisitor.push(SampleData.java:502)
at com.oracle.javafx.authoring.util.Utils$ComponentPathEventHandlerWrapper.push(Utils.java:2086)
at com.oracle.javafx.authoring.util.Utils.visit(Utils.java:2151)
at com.oracle.javafx.authoring.runtime.SampleData.showHideSampleDataRecursively(SampleData.java:513)
at com.oracle.javafx.authoring.runtime.ModelTracker.showHideSampleDataRecursively(ModelTracker.java:652)
at com.oracle.javafx.authoring.runtime.ModelTracker$ModelTransactionListener.unknownChanges(ModelTracker.java:611)
at com.oracle.javafx.authoring.model.transactions.TransactionManager.notifyUnknownChanges(TransactionManager.java:151)
at com.oracle.javafx.authoring.model.transactions.Transaction.close(Transaction.java:262)
at com.oracle.javafx.authoring.stage.v2.gesture.MoveReparentOperationV2.handleMouseReleasedEvent(MoveReparentOperationV2.java:282)
at com.oracle.javafx.authoring.stage.v2.gesture.SelectAndMoveGestureV2.handleMouseEvent(SelectAndMoveGestureV2.java:79)
at com.oracle.javafx.authoring.stage.v2.EditModeController.handleMouseEvent(EditModeController.java:215)
at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:128)
at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:125)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3324)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3164)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3119)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1559)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2261)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:228)
... 2 more