Steps to reproduce:

1. Create new scene and add cube
2. Add Rigidbody
3. Drag cube to make a prefab
4. Remove Rigidbody from prefab
5. In scene select prefab instance and add Rigidbody
6. Press Apply
7. Hold undo shortcut (cmd +z) to perform all possible undo sequence and notice rootTransform != NULL assertion is hit
8. Hold redo shortcut (cmd +shift + z) to perform all possible redo sequence and notice that cube gets duplicated
9. Perform step 7 again ant Unity should crash. If it does not - repeat steps 7 - 8 for a few times until it does

Reproduced with: 5.4.3p2, 5.4.4p4, 5.5.2p1, 5.6.0b10, 2017.1.0a1
Not reproducible: 5.4.0p2 (though assert is still thrown)