Discovered in case 1158222.

Steps to Reproduce :

* Download the attached project
* Open the InspectorWindow and ensure it remains visible
* Select the asset in the Project Window named "SELECT THEN DELETE THIS"
* Delete the asset

Actual Result:
NullReferenceException occurs

Expected Result:
Animation Override Controller gets deleted gracefully

After this change - https://ono.unity3d.com/unity/unity/changeset/18bedf427491dea2281344bd32e6236f36a08324 - asset changes will retain the project window selection. However, if the change is a deletion of the current selection there is a risk that an inspector will try to access a null reference to the old target.

The code linked above makes a call to `GetSceneTracker().ForceReloadInspector()` which leads to us calling

internal virtual void OnForceReloadInspector()
{
if (m_SerializedObject != null)
{
m_SerializedObject.SetIsDifferentCacheDirty();
// Need to make sure internal target list PPtr have been updated from a native memory
// When assets are reloaded they are destroyed and recreated and the managed list does not get updated
// The m_SerializedObject is a native object thus its targetObjects is a native memory PPtr list which have the new PPtr ids.
InternalSetTargets(m_SerializedObject.targetObjects);
}
}

The new targets list will be a non-empty array of null objects. If we have a custom inspector that implements OnDisable, we may run into NullReferenceExceptions if we're missing checks. We will modify the Animation Override Controller's inspector, but customers may start running into exceptions if this isn't fixed at the source.

This issue may be better assigned to the Editor team.