java.lang.AssertionError
   at org.netbeans.modules.form.layoutdesign.LayoutModel.addInterval(LayoutModel.java:322)
   at org.netbeans.modules.form.layoutdesign.LayoutEvent$Interval.undoIntervalRemoval(LayoutEvent.java:226)
   at org.netbeans.modules.form.layoutdesign.LayoutEvent$Interval.undo(LayoutEvent.java:168)
   at org.netbeans.modules.form.layoutdesign.LayoutModel.undo(LayoutModel.java:889)
   at org.netbeans.modules.form.layoutdesign.LayoutModel$LayoutUndoableEdit.undo(LayoutModel.java:948)
   at javax.swing.undo.CompoundEdit.undo(CompoundEdit.java:63)
   at javax.swing.undo.UndoManager.undoTo(UndoManager.java:335)
   at javax.swing.undo.UndoManager.undo(UndoManager.java:413)
   at org.openide.awt.UndoRedo$Manager.undo(UndoRedo.java:184)
   at org.netbeans.modules.form.FormModel$UndoRedoManager.superUndo(FormModel.java:728)
   at org.netbeans.modules.form.FormModel$UndoRedoManager.undo(FormModel.java:737)
   at org.openide.actions.UndoRedoAction.actionPerformed(UndoRedoAction.java:220)
   at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
   at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
   at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
   at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
   at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
   at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
   at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:289)
   at java.awt.Component.processMouseEvent(Component.java:6504)
   at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
   at java.awt.Component.processEvent(Component.java:6269)
   at java.awt.Container.processEvent(Container.java:2229)
   at java.awt.Component.dispatchEventImpl(Component.java:4860)
   at java.awt.Container.dispatchEventImpl(Container.java:2287)
   at java.awt.Component.dispatchEvent(Component.java:4686)
   at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4832)
   at java.awt.LightweightDispatcher.processMouseEvent(LightweightDispatcher.java:4492)
   at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:4422)
   at java.awt.Container.dispatchEventImpl(Container.java:2273)
   at java.awt.Window.dispatchEventImpl(Window.java:2713)
   at java.awt.Component.dispatchEvent(Component.java:4686)
   at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
   at java.awt.EventQueue.access$000(EventQueue.java:101)
   at java.awt.EventQueue$3.run(EventQueue.java:666)
   at java.awt.EventQueue$3.run(EventQueue.java:664)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
   at java.awt.EventQueue$4.run(EventQueue.java:680)
   at java.awt.EventQueue$4.run(EventQueue.java:678)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
   at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:148)
   at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
   at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
   at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
   at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)