java.lang.NullPointerException
        at javax.swing.AbstractButton.removeNotify(AbstractButton.java:1013)
        at javax.swing.JButton.removeNotify(JButton.java:217)
        at java.awt.Container.removeNotify(Container.java:2619)
        at javax.swing.JComponent.removeNotify(JComponent.java:4699)
        at java.awt.Container.removeNotify(Container.java:2619)
        at javax.swing.JComponent.removeNotify(JComponent.java:4699)
        at java.awt.Container.removeNotify(Container.java:2619)
        at javax.swing.JComponent.removeNotify(JComponent.java:4699)
        at java.awt.Container.removeNotify(Container.java:2619)
        at javax.swing.JComponent.removeNotify(JComponent.java:4699)
        at java.awt.Container.removeNotify(Container.java:2619)
        at javax.swing.JComponent.removeNotify(JComponent.java:4699)
        at javax.swing.JRootPane.removeNotify(JRootPane.java:750)
        at java.awt.Container.removeNotify(Container.java:2619)
        at java.awt.Window.removeNotify(Window.java:650)
        at java.awt.Frame.removeNotify(Frame.java:879)
        at java.awt.Window$1DisposeAction.run(Window.java:978)
        at java.awt.Window.doDispose(Window.java:990)
        at java.awt.Window.dispose(Window.java:937)
        at javax.swing.JFrame.processWindowEvent(JFrame.java:287)
        at java.awt.Window.processEvent(Window.java:1785)
        at java.awt.Component.dispatchEventImpl(Component.java:4413)
        at java.awt.Container.dispatchEventImpl(Container.java:2116)
        at java.awt.Window.dispatchEventImpl(Window.java:2440)
        at java.awt.Component.dispatchEvent(Component.java:4243)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:104)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)