We get this error after using "Debug/Fix" from the debugger, and then

Clear All

in the command window.

No way to get around this found yet.
We just use "Debug/Fix" from the debugger, after that

Set sysmenu to default

in the command window (in order to be able to cut/copy/paste with keyboard shortcuts), correct the error, save the form/class and close it.
And after that we use

Clear All

...and all works fine... :o|