Steps to reproduce:
1. Open users attached project
2. Press Play
3. In Assets folder select "Drop" Prefab
4. Click Open Prefab in the Inspector window
5. Change the speed parameter and go back to normal Hierarchy window

Expected results: No errors are thrown after script parameter of the Prefab is edited during the Play mode
Actual results: After any parameter is edited in the script of the Prefab during Play mode, error will appear "MissingReferenceException: The object of type 'Drop' has been destroyed but you are still trying to access it." Your script should either check if it is null or you should not destroy the object.

Reproducible with: 2018.3.0a6, 2018.3.0f2, 2019.1.0a11
Not reproducible with: 2017.4.16f1, 2018.2.18f1, 2018.3.0a5

Note: Improved Prefabs appeared in 2018.3.0a6. This error will only appear on Instantiated GameObject.