Got the project, thanks. I see the same crash and the state of the objects, though I'm not clear how they could get in that state. I tried various steps to reproduce (duplicate bones with slots, etc) but couldn't make it happen. I added some code that should fix it, though without being able to reproduce it I can't be sure.
Edit: Turns out these steps cause the issue:
viewtopic.php?f=3&t=752
Verified it is fixed for sure! 
Awesome Job! Onion skinning is a great feature. Loving it!
Yay! I've been looking forward to this one. Great looking feature! Well done!
hi, i searching for a onion feature, and Gosting are what i need.
But i can not find the button in my pro version ? 3.6.52
How its work and how i can show ghost(ognion) interpolation. ?
link documentation plz or video
thank a lot for help
EDIT: OK SORRY I FIND IT
Ghosting - Spine User Guide
