"Invalid sorting group ID" assert when removing a sorting group attached to a prefab GameObject that is the parent of another GameObject with a different sorting group

1. Open scene "1" in attached project
2. In hierarchy, click on Body
3. In Inspector, remove its Sorting Group component

This error will show in the Console:

Invalid sorting group ID
UnityEditor.DockArea:OnGUI()

Any further keyboard/mousebutton input will cause the same error(s) to show up in the console.