JSlider error, but no clue where it's coming from

Question

At some point in my program, the following error is being generated. Any idea what this is?

  Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicSliderUI.calculateThumbLocation(Unknown Source)
        at javax.swing.plaf.basic.BasicSliderUI.calculateGeometry(Unknown Source)
        at javax.swing.plaf.basic.BasicSliderUI$Handler.componentResized(Unknown Source)
        at java.awt.Component.processComponentEvent(Unknown Source)
        at java.awt.Component.processEvent(Unknown Source)
        at java.awt.Container.processEvent(Unknown Source)
        at java.awt.Component.dispatchEventImpl(Unknown Source)
        at java.awt.Container.dispatchEventImpl(Unknown Source)
        at java.awt.Component.dispatchEvent(Unknown Source)
        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.awt.EventQueue.access$000(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
        at java.awt.EventQueue$4.run(Unknown Source)
        at java.awt.EventQueue$4.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

I had similar intermittent NPEs pop up during initialization when setting a slider to use a custom ui that extended BasicSliderUI. I fixed them by wrapping it in an invokeLater:

SwingUtilities.invokeLater(new Runnable(){
        @Override public void run() {
            slider.setUI(new CustomSliderUI(slider));
        }
    });