How to reproduce:
1. Download and open the submitted project
2. Double click on the Cube Prefab to open it in Prefab edit mode
3. Open Cube2 child of the Cube Prefab in the Prefab edit mode
4. Delete the Cube Prefab
5. Press the arrow in the upper-left corner of the Hierarchy window

Expected result: Editor goes back to the Scene edit mode
Actual result: Editor throws a Console error

Reproduced with: 2018.4.3f1, 2019.1.8f1, 2019.2.0b7, 2019.3.0a7

Notes:
Prefab edit mode did not exist until Unity version 2018.3.0a6, therefore the issue was not reproducible.