You are getting the stack because the toolbar model has an invalid entry. Why does it have an invalid entry? I do not know. When this happens, most of the time anyway, it is caused by something that happened 5 minutes/1 hour/3 days ago, and is very difficult to track down without a reproducable set of steps.
Reset your toolbars from the SlickEdit command line:
tbResetAll
If you have a set of steps that reproduces the issue, then please let support@slickedit.com know. And, of course, if the above does not solve your problem, then let us know that too.
--rodney