java.lang.NullPointerException
        at org.netbeans.modules.uml.diagrams.UMLRelationshipDiscovery.createNestedLink(UMLRelationshipDiscovery.java:574)
        at org.netbeans.modules.uml.diagrams.UMLRelationshipDiscovery.createNestedLinks(UMLRelationshipDiscovery.java:400)
        at org.netbeans.modules.uml.diagrams.UMLRelationshipDiscovery.createdNestedLinks(UMLRelationshipDiscovery.java:494)
        at org.netbeans.modules.uml.diagrams.UMLRelationshipDiscovery.discoverCommonRelations(UMLRelationshipDiscovery.java:138)
        at org.netbeans.modules.uml.drawingarea.actions.SceneAcceptProvider.createConnection(SceneAcceptProvider.java:594)
        at org.netbeans.modules.uml.drawingarea.actions.SceneAcceptProvider.accept(SceneAcceptProvider.java:529)
        at org.netbeans.modules.uml.drawingarea.actions.SceneAcceptProvider.accept(SceneAcceptProvider.java:205)
        at org.netbeans.modules.uml.drawingarea.actions.DiagramSceneAcceptAction.drop(DiagramSceneAcceptAction.java:132)
        at org.netbeans.api.visual.action.WidgetAction$Chain.drop(WidgetAction.java:1075)
        at org.netbeans.api.visual.widget.SceneComponent$Operator$18.operate(SceneComponent.java:651)
        at org.netbeans.api.visual.widget.SceneComponent.processLocationOperator(SceneComponent.java:338)
        at org.netbeans.api.visual.widget.SceneComponent.processLocationOperator(SceneComponent.java:287)
        at org.netbeans.api.visual.widget.SceneComponent.drop(SceneComponent.java:246)
        at java.awt.dnd.DropTarget.drop(DropTarget.java:434)
        at sun.awt.dnd.SunDropTargetContextPeer.processDropMessage(SunDropTargetContextPeer.java:500)
        at sun.awt.X11.XDropTargetContextPeer.processDropMessage(XDropTargetContextPeer.java:165)
        at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchDropEvent(SunDropTargetContextPeer.java:812)
        at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(SunDropTargetContextPeer.java:736)
        at sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:30)
        at java.awt.Component.dispatchEventImpl(Component.java:4440)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4413)
        at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4556)
        at java.awt.LightweightDispatcher.processDropTargetEvent(LightweightDispatcher.java:4291)
        at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:4145)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2475)
        at java.awt.Component.dispatchEvent(Component.java:4413)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:104)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)