To reproduce:
1) Drop an AnchorPane
2) Drop a GridPane
3) Delete the three columns one by one
4) Drop a Button in the empty GridPane using Hierarchy
5) Drag another Button on the right edge of the GridPane in Content panel
    => SB crashes with the following stack


java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.job.gridpane.v2.InsertColumnJob.<init>(InsertColumnJob.java:71)
at com.oracle.javafx.scenebuilder.kit.editor.drag.target.GridPaneDropTarget.makeDropJob(GridPaneDropTarget.java:223)
at com.oracle.javafx.scenebuilder.kit.editor.drag.DragController.end(DragController.java:113)
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$8.handle(LibraryPanelController.java:668)
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$8.handle(LibraryPanelController.java:663)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$DnDGesture.processDropEnd(Scene.java:3062)
at javafx.scene.Scene$DnDGesture.access$6800(Scene.java:2896)
at javafx.scene.Scene$DragSourceListener.dragDropEnd(Scene.java:3306)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$6.run(GlassSceneDnDEventHandler.java:150)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$6.run(GlassSceneDnDEventHandler.java:145)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragEnd(GlassSceneDnDEventHandler.java:145)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragEnd(GlassViewEventHandler.java:615)
at com.sun.glass.ui.View.handleDragEnd(View.java:652)
at com.sun.glass.ui.View.notifyDragEnd(View.java:966)
at com.sun.glass.ui.mac.MacPasteboard._putItemsFromArray(Native Method)
at com.sun.glass.ui.mac.MacPasteboard.putItemsFromArray(MacPasteboard.java:143)
at com.sun.glass.ui.mac.MacPasteboard.putItems(MacPasteboard.java:165)
at com.sun.glass.ui.mac.MacSystemClipboard.pushToSystem(MacSystemClipboard.java:223)
at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:274)
at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1150)
at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2940)
at javafx.scene.Scene$DnDGesture.process(Scene.java:3009)
at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2896)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3762)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
at com.sun.glass.ui.View.notifyMouse(View.java:898)