If you are editing a prefab with a capsule collider attached and then discard the changes you made by leaving isolation mode, the following exception will occur:

MissingReferenceException: The object of type 'CapsuleCollider' 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.

As a knockon, the next time you select the prefab or any instance of it in the hierarchy, several more exceptions will occur.

1. Install Unity
2. Open the attached project
3. Find GameObjectWithCapsuleCollider.prefab in the assets folder
4. Open the prefab in isolation mode
5. Make sure that autosave is turned OFF
6. Move the transform
7. Exit isolation mode and discard all changes

Expected Result:
The changes are discarded and no exceptions occur.

Actual Result:
A MissingReferenceException occcurs.

Tested on Windows.

Occurs on 2019.2.0a1, 2019.1.0a14

Does not occur on 2018.3.0f2, 2019.1.0a13