Drop a GridPane.
Drop a Button in cell 1,1.
From the Layout panel, uncheck the columnIndex value (to set it to null) : the Button is moving in cell 1,0.
Move back the Button (from the Content View) to cell 1,1 : the Content View display is freezed, a NPE is raised (see below). One need to select again the button to have the Content View displayed correctly. The tool is still usable.

java.lang.NullPointerException
at com.oracle.javafx.authoring.droptargetinfo.GridPaneDropTargetInfo.checkChildIndexes(GridPaneDropTargetInfo.java:194)
at com.oracle.javafx.authoring.droptargetinfo.GridPaneDropTargetInfo.containsComponents(GridPaneDropTargetInfo.java:83)
at com.oracle.javafx.authoring.stage.v2.gesture.MoveReparentOperationV2.handleMouseReleasedEvent(MoveReparentOperationV2.java:194)
at com.oracle.javafx.authoring.stage.v2.gesture.SelectAndMoveGestureV2.handleMouseEvent(SelectAndMoveGestureV2.java:76)
at com.oracle.javafx.authoring.stage.v2.EditModeController.handleMouseEvent(EditModeController.java:216)
at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:100)
at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:97)
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)