Reproduction steps:
1. Open user's attached project "2DAnim_20_1_0a25_BurstCollectionsTest"
2. From the Project window, select "Fei_Cel_group" and open it in Sprite Editor
3. Make sure Skinning Editor mode is selected
4. Add a bone to the character using skinning editor, click "Apply"
5. Install Burst and Collections packages
6. Remove Burst and Collections packages
7. Open "Fei_Cel_group" in the Skinning Editor again
8. Remove the created bone and click "Apply"
-- Observe crash

Reproducible with: 2019.3.6f1, 2020.1.0b2, 2020.2.0a3
Could not test with: 2017.4.38f1(no Package Manager), 2018.4.19f1 (asset broke after downgrade)

Reproducible with package versions: 3.0.8, 4.1.0, 4.1.1