Reproduction steps:
1. Open the attached project "2022.2.0a11-packageRepro.zip"
2. Open Scenes/SampleScene
3. Select the Assets/Fei-CLONE sprite in the Project window
4. Open Sprite Editor in the Inspector window
5. Open Skinning Editor in the Sprite Editor window
6. Make some edits with the Skinning Editor

Expected result: Changes made are saved
Actual result: Editor crashes

*Reproducible:*
* Unity 2022.2.0a16 + 2D Animation 9.0.0-pre.2: Crashes
* Unity 2022.2.0a16 + 2D Animation 9.0.0-pre.1: Crashes
* Unity 2022.2.0a16 + 2D Animation 8.0.0: Crashes

*Not reproducible:*
* Unity 2022.1.0f1 + 2D Animation 8.0.0: No crash
* Unity 2022.1.0f1 + 2D Animation 9.0.0-pre.2: No Crash (I had to do minor package modifications to have it compile with the older editor version)