Adding a component to the root of a fbx prefab with an assetPostProcessor, causes the inspector to throw a null reference exception, if the fbx prefab is selected while it is re-imported (caused by the model fbx changing on disk or by changing the animation type).

To reproduce:
1) Import attached package into Unity project
2) Select the imported model.
3) In Rig section, change animation type into Humanoid

Argument exceptions are seen upon the change.

However, if you clear console and simply reimport, the errors don't reappear.

Reproduced in 2017.1.0f3, 2017.3.0p1, 2017.1.0b2