Oh Ok!
Here is the problem report the mac generates. It has something like what you mentioned
"Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Can only send messages on the main thread'"

I am glad to hear it is fixed in v3.

Thanks!

Side note:
After some more poking, I notice this only happens after fullscreening. And it doesn't always involve undo. But it doesn't crash when NOT in fullscreened. Weird. I have 2 screens on the mac right now (the laptop screen and an external monitor). Let me see if it does it when only 1 screen is attached...
Still happens even if only using the mac laptop screen and still only after fullscreen.

Please find attached my spine.log

System: MacBook Air, El Capitan

Steps to recreate:
1) Open Spine pro demo (spineboy) project
2) fiddle with rotating stuff
3) mash cmd+Z repeatedly
Effect: Crashes and Closes, get mac system crash report.

Note: This also happens frequently and sometimes even when I don't cmd+z, but cmd+z seems to be the thing that gets it to happen the quickest. but no matter what I do it happens within ₃ minutes of use of just rotating and fiddling.

Thanks!

Edit in case it is helpful, Chris (my workmate) doesn't experience this on his windows machine. Only on our macbook air.