NPE from MultiUIDefaults.getUIError

When running the unit test

http://hg.netbeans.org/core-main/file/69be2d4eadf2/openide.loaders/test/unit/src/org/openide/awt/MenuBarNotInAWTTest.java

in JDK 6u18, the following stack trace is printed (though the test completes normally):

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.MultiUIDefaults.getUIError(MultiUIDefaults.java:133)
        at javax.swing.UIDefaults.getUI(UIDefaults.java:741)
        at javax.swing.UIManager.getUI(UIManager.java:989)
        at javax.swing.JMenuBar.updateUI(JMenuBar.java:127)
        at org.openide.awt.MenuBar.updateUI(MenuBar.java:168)
        at org.openide.awt.MenuBar$1.run(MenuBar.java:155)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        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)

I could try to produce a minimal standalone test case if that is required.