Reproduction steps:
1. Open the project "RigComponentBug"
2. Open Scenes/SampleScene
3. Select the "Cube" GameObject
4. In the Inspector, click "Add Component" and add the "RigComponent" Script

Expected result: The "Bones" field is displayed
Actual result: The "Bones" field is not displayed

Reproducible with: 0.6.0-preview.2, 0.9.0-preview.6 (2020.3.24f1, 2020.3.32f1)
Not reproducible with: 0.6.0-preview.2, 0.9.0-preview.6 (2020.3.23f1, 2021.2.17f1)
Could not test with: 0.5.1-preview.3 (2019.4.36f1, "Bones" field is displayed as an empty list), 0.9.0-preview.6 (2022.1.0b14, 2022.2.0a9, Getting compilation errors: S8377: The type 'U' must be a non-nullable value type)