Editor crashed on removing Rigidbody from override window while it was required for HingeJoint component.

Steps to reproduce:
1. Open Unity
2. Create a GameObject.
3. Make a prefab out of it
4. Add hinge Joint
5. Click on Overrides
6. Revert added Rigidbody (Click on RigidBody Override, then Click "Revert" button)
7. ApplyAll

Actual Result: Editor crashes.

Expected Result: Dependent components should not be removed/reverted separately from Override window.

Reproducible On: 2019.1.0a6, 2018.3.0b7, 2018.3.0a6

Notes:
- Improved Prefabs were added in 2018.3.0a6
- Appears on both Mac and Windows.