How to reproduce:
1. Open the attached project
2. Open "Bug" scene
3. Open Scene view and select "Char" in Hierarchy
4. In the Scene view press "Enable Preview" window
5. Make some changes wherever in editor and press "Ctrl+Z"
6. You will get an exception NullReferenceException: Object reference not set to an instance of an object
7. Open Animation window and do the same steps 3-5
8. Notice that Error message doesn't come up, you can even close animation window after that and do same steps error will not be thrown
9. Reimport "Bug.cs" and do the same steps
10. You will get error again