Here's the thread:
  http://forums.java.net/jive/post!reply.jspa?messageID=325828

Exception with this test:
#>/cygdrive/m/re/1.6.0_12/promoted/all/b01/binaries/windows-i586/bin/java TestJPanel
isValid
Exception in thread "main" java.lang.NullPointerException
        at TestJPanel.isValid(TestJPanel.java:18)
        at java.awt.Component.invalidateIfValid(Component.java:2707)
        at java.awt.Component.setLocale(Component.java:1777)
        at javax.swing.JComponent.<init>(JComponent.java:595)
        at javax.swing.JPanel.<init>(JPanel.java:65)
        at javax.swing.JPanel.<init>(JPanel.java:92)
        at javax.swing.JPanel.<init>(JPanel.java:100)
        at TestJPanel.<init>(TestJPanel.java:7)
        at TestJPanel.main(TestJPanel.java:22)