To reproduce:
1) Open the attached file (sample.fxml)
2) Drag a button and drop it to the root GridPane in Hierarchy
=> the following exception is thrown and the tool is blocked:


2013-05-16 16:38:41.245 java[65006:707] [JRSAppKitAWT markAppIsDaemon]: Process manager already initialized: can't fully enable headless mode.
java.lang.AssertionError
   at com.oracle.javafx.authoring.stage.v2.handles.gridpane.GridPaneMosaic.getTile(GridPaneMosaic.java:285)
   at com.oracle.javafx.authoring.stage.v2.handles.gridpane.GridPaneMosaic.selectCell(GridPaneMosaic.java:405)
   at com.oracle.javafx.authoring.stage.v2.handles.gridpane.GridPaneDropTargetRing.paintTargetHighlight(GridPaneDropTargetRing.java:163)
   at com.oracle.javafx.authoring.stage.v2.handles.gridpane.GridPaneDropTargetRing.createGridDividers(GridPaneDropTargetRing.java:139)
   at com.oracle.javafx.authoring.stage.v2.handles.gridpane.GridPaneDropTargetRing.createChildren(GridPaneDropTargetRing.java:60)
   at com.oracle.javafx.authoring.stage.v2.handles.ring.GenericDropTargetRing.update(GenericDropTargetRing.java:48)
   at com.oracle.javafx.authoring.stage.v2.handles.TrackerV5.updateNow(TrackerV5.java:341)
   at com.oracle.javafx.authoring.stage.v2.handles.TrackerV5.access$200(TrackerV5.java:30)
   at com.oracle.javafx.authoring.stage.v2.handles.TrackerV5$4.run(TrackerV5.java:93)
   at com.oracle.javafx.authoring.util.Utils$NextFrameTimer.handle(Utils.java:549)
   at com.sun.scenario.animation.AbstractMasterTimer.timePulseImpl(AbstractMasterTimer.java:376)
   at com.sun.scenario.animation.AbstractMasterTimer$MainLoop.run(AbstractMasterTimer.java:289)
   at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:456)
   at com.sun.javafx.tk.quantum.QuantumToolkit$9.run(QuantumToolkit.java:329)
   at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)