How to reproduce:
1. Open the attached project
2. Select panda sprite and open Sprite Editor
3. Select one of the pandas and switch to Bone Editor
4. Select Create Bone and start adding bones
5. Create few bones on panda and click Escape
6. Select Create Free Bone and add few bones
7. Undo all stelps with CMD+Z till only circle of bones is left
8. Repeat 4-7 steps
Result: after clicking Undo project freezes