This did not occur in build 11 of Mantis, but it does occur in build 12.

To reproduce:

Run the Animator demo applet in appletviewer.
Reload the applet.
The AppletViewer window disappears.
Eventually, the animation (sounds) start again, and the following exception
is thrown:

/java/re/jdk/1.4.2/promoted/beta/b12/binaries/solaris-sparc/bin/appletviewer example1.html
java.lang.NullPointerException
        at Animation.loadAnimationMedia(Animator.java:574)
        at Animation.init(Animator.java:503)
        at Animator.init(Animator.java:281)
        at sun.applet.AppletPanel.run(AppletPanel.java:352)
        at java.lang.Thread.run(Thread.java:536)

I don't know if this also happens when reloading an applet in a browser or not.

It is probably introduced by the changes the plugin team made to
AppletViewer.java (regarding handling quit signal) in build 12.