If it crashes when you try to IWT but have no target, that would definitely be a WoW bug to report, and it would explain to me why people would get it intermittently with ISBoxer, but not typically without.

Here's why...

The inherent delay in /assist, as diagrammed here from http://isboxer.com/wiki/Game_features#Assist
Assist_delay.jpg

.. will increase the odds that your slave windows will randomly crash due to attempting to assist. You would get this crash intermittently on slaves when you select a mob and immediately press IWT. The longer you have your target before pressing IWT, the safer it would be.