How to reproduce:
1. Open the attached project "1249713.zip"
2. Open the "Test" Scene
3. In the menu bar click "BugTest" > "Test bug"
4. Double click any Prefab in the Project window

Expected result: the Scene View does not switch to Prefab Mode and the Prefab is not opened
Actual result: Prefab is not opened, however, the Scene View switches to Prefab Mode, the Hierarchy window displays empty Prefab header bar and "Not a prefab scene" error appears in the Console

Reproducible with: 2018.4.24f1, 2019.4.1f1, 2020.1.0a4
Not reproducible with: 2020.1.0a5, 2020.2.0a15

Note:
- 2020.1.0a5 and above throws System.NullReferenceException error to the Console as the Test.cs script tries to modify undefined Object - this is expected