How to reproduce:
1. Open the attached project ("case_1122560-PrefabNull.zip")
2. Open the "Bug" Scene and click on the "Main" object in the Hierarchy
3. In the Inspector Right-click the "Prefab Bug Demonstration" script and select "Test Prefab"
4. Observe the Console Window
5. Open the "Prefab" object and change the "A field to Edit" value to "Test"
6. Close the prefab mode and repeat steps 2&3
7. Observe the Console Window again

Expected result: The prefab is returned as non-null and a debug message "Value of prefab is not null Prefab" is shown
Actual result: The prefab is returned as null and a debug message "Value of prefab is null" is shown

Reproduced in: 2019.2.0a4, 2019.1.0b2, 2018.3.4f1, 2018.3.0a6
Not reproducible in: 2017.4.19f1
Couldn't test with: 2018.3.0a5 (Breaks the prefab scripts and the improved prefabs were yet to be implemented)

Note: Entering Play mode fixes the problem and the script returns the prefab as non-null again