Bug 10774 (NPE in ToolTipManager.mouseExited) has popped up
here twice in two days. You may also have seen it. It has a couple
I decided to investigate. 10774 is caused by a bug in Swing,
This bug is marked as "Closed, not reproducible", which means
that they are unable to reproduce it in JDK 1.5.
A workaround for this bug is to override removeNotify and send
a fake MouseEvent to ToolTipManager.
This will hide whatever tooltip is showing, preventing mouseExited
from accessing an invalid window later on.