Steps to reproduce:
1. Open user's attached project "19.3.zip"
2. Open "1" scene
3. In the Hierarchy window select "GameObject"
4. Try dragging "Source" or "Target" GameObject from the Hierarchy to GameObject script component under IClickable List
5. Notice that this action has no effect
6. Try changing the List Size
7. Notice the error in the console "Generating diff of this object for undo because the type tree changed."

Expected results: GamesObjects are added in the Interface IClickble list in the Inspector window
Actual results: GamesObjects can't be added in the Interface IClickable list in the Inspector window and an error is thrown

Reproducible with: 2019.3.0a2, 2019.3.0b2, 2020.1.0a3

Notes: Most of the time error is not thrown. In the Unity version 2019.3.a1 and lower [SerializeReference] can't be accessed due to its protection level making the public IClickable list invisible making the situation impossible to test