How to reproduce:
1. Create a new project
2. Create a prefab
3. Open it in the Prefab mode
4. Duplicate the prefab
5. Delete the root prefab from assets
6. Rename duplicated prefab same as root prefab was named

Expected result: Prefab Mode doesn't show missing prefab
Actual result: Hierarchy and Scene tabs get corrupted

Reproducible: 2018.4.2f1, 2019.1.6f1, 2019.2.0b5
Not reproducible: 2019.3.0a1, 2019.3.0a5
Can't reproduce: 2017.4, because nested prefabs were not introduced

Workaround: reopen the project