Reproduction steps:
1. Open user's project "TestingProject.zip"
2. Open Assets/ArmTest folder
3. Notice that "Arm_Rig" is using "Arm_Rig_Base" Skeleton Asset, which means it's dependent on it
4. Click on "Arm_Rig_Base"
5. In the Inspector window click on the Sprite Editor button
6. Choose Skinning Editor in the dropdown list in the Top-Left corner
7. Select Edit Bone mode and click on any bone
8. Press the Delete button on your keyboard and click on the Apply button
9. Close Sprite Editor window
10. Click on "Arm_Rig" and in the Inspector window click on the Sprite Editor button
11. Choose Skinning Editor in the dropdown list in the Top-Left corner

Expected result: Some type of warning window about broken parent Skeleton Asset is shown and exceptions are handled
Actual result: Exceptions are thrown and GUI Errors are printed out to the Console without useful indicator of what has gone wrong

Reproducible with: 5.0.0-pre.1 (2021.1.0b1, 2021.1.0b3, 2021.2.0a2)
Couldn't test with: All other versions because 5.0.0-pre.1 is not available or breaks the project, and 5.0.0-pre.1 is needed because it added bone sharing from other PSDImporter file functionality