Using an AssetPostProcessor to add a Component to an imported model (fbx), causes Unity to display an error when import settings changes are applied afterwards.

STR:
1. Open user provided project
2. Select Assets/Art/Box.fbx
3. Toggle "Optimize Mesh" option (can be any option)
4. Press "Apply" button

Notice that errors are being thrown in the console:
MissingReferenceException: The object of type 'MyScript' 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.

Reproduced with: 5.3.5f1, 5.4.0f3, 5.5.0b5

Workaround - Unselect/reselect the fbx.