java.awt.IllegalComponentStateException: contentPane cannot be set to null.
   at javax.swing.JRootPane.setContentPane(JRootPane.java:620)
   at javax.swing.JFrame.setContentPane(JFrame.java:693)
   at org.netbeans.core.windows.EditorOnlyDisplayer.cancel(EditorOnlyDisplayer.java:153)
   at org.netbeans.core.windows.view.ui.slides.SlideBar.stateChanged(SlideBar.java:255)
   at javax.swing.DefaultSingleSelectionModel.fireStateChanged(DefaultSingleSelectionModel.java:132)
   at javax.swing.DefaultSingleSelectionModel.setSelectedIndex(DefaultSingleSelectionModel.java:67)
   at org.netbeans.core.windows.view.ui.slides.TabbedSlideAdapter.setSelectedComponent(TabbedSlideAdapter.java:296)
   at org.netbeans.core.windows.view.ui.TabbedHandler.setSelectedTopComponent(TabbedHandler.java:275)
   at org.netbeans.core.windows.view.ui.AbstractModeContainer.setSelectedTopComponent(AbstractModeContainer.java:118)
   at org.netbeans.core.windows.view.ui.AbstractModeContainer.setTopComponents(AbstractModeContainer.java:131)
   at org.netbeans.core.windows.view.ui.slides.SlideBarContainer.setTopComponents(SlideBarContainer.java:126)
   at org.netbeans.core.windows.view.ModeView.setTopComponents(ModeView.java:138)
   at org.netbeans.core.windows.view.ViewHierarchy.updateViewForAccessor(ViewHierarchy.java:269)
   at org.netbeans.core.windows.view.ViewHierarchy.updateSlidingViews(ViewHierarchy.java:380)
   at org.netbeans.core.windows.view.ViewHierarchy.updateViewHierarchy(ViewHierarchy.java:179)
   at org.netbeans.core.windows.view.DefaultView.changeGUI(DefaultView.java:156)
   at org.netbeans.core.windows.ViewRequestor.dispatchRequest(ViewRequestor.java:275)
   at org.netbeans.core.windows.ViewRequestor.processRequest(ViewRequestor.java:251)
   at org.netbeans.core.windows.ViewRequestor.postRequest(ViewRequestor.java:189)
   at org.netbeans.core.windows.ViewRequestor.scheduleRequest(ViewRequestor.java:127)
   at org.netbeans.core.windows.Central.activateModeTopComponent(Central.java:1797)
   at org.netbeans.core.windows.WindowManagerImpl.topComponentRequestActive(WindowManagerImpl.java:1334)
   at org.openide.windows.TopComponent.requestActive(TopComponent.java:861)
   at org.netbeans.core.io.ui.IOWindow$IOWindowImpl.requestActive(IOWindow.java:301)
   at org.netbeans.core.io.ui.IOWindow.requestActive(IOWindow.java:125)
   at org.openide.windows.IOContainer.requestActive(IOContainer.java:125)
   at org.netbeans.core.output2.Controller.performCommand(Controller.java:340)
   at org.netbeans.core.output2.Controller.eventDispatched(Controller.java:108)
   at org.netbeans.core.output2.IOEvent.dispatch(IOEvent.java:281)
   at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
   at java.awt.EventQueue.access$200(EventQueue.java:103)
   at java.awt.EventQueue$3.run(EventQueue.java:694)
   at java.awt.EventQueue$3.run(EventQueue.java:692)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
   at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:159)
   at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
   at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
   at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
   at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)