Probably a long shot but if anyone has an idea.
I use ClipTrack::insertWaveClip to insert several audio clips on separate tracks.
When I undo/redo rapidly, I eventually get a crash here:
‘async’ is suspect. It always ends up attempting to do things too late, like after an object that it is referencing has already been destroyed.
In your release build, does everything work properly? Perhaps the jassert poses no problem and in your case only means that an undo action has been discarded. Or are there some other side-effects?
It could be that JUCE assumes that if perform() is called while still performing an undo/redo, it must be due to perform() being called recursively (which you are not doing), and the jassert is just there to warn people not to do that.