java.lang.NullPointerException
        at org.netbeans.modules.uml.drawingarea.persistence.PersistenceManager.hasContainerWidgetAsParent(PersistenceManager.java:133)
        at org.netbeans.modules.uml.drawingarea.persistence.PersistenceManager.saveDiagram(PersistenceManager.java:94)
        at org.netbeans.modules.uml.drawingarea.persistence.SaveDiagram.save(SaveDiagram.java:72)
        at org.netbeans.modules.uml.drawingarea.UMLDiagramTopComponent.canClose(UMLDiagramTopComponent.java:406)
        at org.netbeans.core.windows.Central.removeModeTopComponent(Central.java:925)
        at org.netbeans.core.windows.Central.userClosedTopComponent(Central.java:1946)
        at org.netbeans.core.windows.view.DefaultView.userClosedTopComponent(DefaultView.java:741)
        at org.netbeans.core.windows.view.ui.TabbedHandler.actionPerformed(TabbedHandler.java:320)
        at org.netbeans.swing.tabcontrol.TabbedContainer.postActionEvent(TabbedContainer.java:692)
        at org.netbeans.swing.tabcontrol.TabbedContainerUI.shouldPerformAction(TabbedContainerUI.java:162)
        at org.netbeans.swing.tabcontrol.plaf.DefaultTabbedContainerUI.access$2700(DefaultTabbedContainerUI.java:109)
        at org.netbeans.swing.tabcontrol.plaf.DefaultTabbedContainerUI$DisplayerActionListener.actionPerformed(DefaultTabbedContainerUI.java:1258)
        at org.netbeans.swing.tabcontrol.TabDisplayer.postActionEvent(TabDisplayer.java:557)
        at org.netbeans.swing.tabcontrol.TabDisplayerUI.shouldPerformAction(TabDisplayerUI.java:185)
        at org.netbeans.swing.tabcontrol.plaf.BasicTabDisplayerUI.access$1500(BasicTabDisplayerUI.java:116)
        at org.netbeans.swing.tabcontrol.plaf.BasicTabDisplayerUI$BasicDisplayerMouseListener.performCommand(BasicTabDisplayerUI.java:718)
        at org.netbeans.swing.tabcontrol.plaf.BasicTabDisplayerUI$BasicDisplayerMouseListener.potentialCommand(BasicTabDisplayerUI.java:703)
        at org.netbeans.swing.tabcontrol.plaf.BasicTabDisplayerUI$BasicDisplayerMouseListener.mouseReleased(BasicTabDisplayerUI.java:751)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
        at java.awt.Component.processMouseEvent(Component.java:6134)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
        at java.awt.Component.processEvent(Component.java:5899)
        at java.awt.Container.processEvent(Container.java:2023)
        at java.awt.Component.dispatchEventImpl(Component.java:4501)
        at java.awt.Container.dispatchEventImpl(Container.java:2081)
        at java.awt.Component.dispatchEvent(Component.java:4331)
        at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4301)
        at java.awt.LightweightDispatcher.processMouseEvent(LightweightDispatcher.java:3965)
        at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:3895)
        at java.awt.Container.dispatchEventImpl(Container.java:2067)
        at java.awt.Window.dispatchEventImpl(Window.java:2458)
        at java.awt.Component.dispatchEvent(Component.java:4331)
        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)