Step to Reproduce:
1) with the latest j2sdk (fcsW) launch SwingSet2, change l&f to Motif
2) go to Internal Frame demo tab
3) click off the Closable and Maximizable checkboxes
4) make a few internal frames, click the Closable and Maximizable checkboxes again a couple of times

should get the exception quite quickly, if not try the sequence a few more times
Result:
exception below
java.lang.NullPointerException
        at com.sun.java.swing.plaf.motif.MotifDesktopPaneUI$MotifDesktopManager.endDraggingFrame(MotifDesktopPaneUI.java:112)
        at javax.swing.plaf.basic.BasicInternalFrameUI$BorderListener.mouseReleased(BasicInternalFrameUI.java:668)
        at java.awt.Component.processMouseEvent(Component.java:3717)
        at java.awt.Component.processEvent(Component.java:3546)
        at java.awt.Container.processEvent(Container.java:1164)
        at java.awt.Component.dispatchEventImpl(Component.java:2595)
        at java.awt.Container.dispatchEventImpl(Container.java:1213)
        at java.awt.Component.dispatchEvent(Component.java:2499)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:2451)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:2216)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:2125)
        at java.awt.Container.dispatchEventImpl(Container.java:1200)
        at java.awt.Window.dispatchEventImpl(Window.java:912)
        at java.awt.Component.dispatchEvent(Component.java:2499)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:319)
        at java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:103)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)

platforms:
win32, (95) solaris 2.7

******************************************************************8

###@###.### 2000-05-16
verified fix on winNT, MerlinB12, bug no longer occurs.