To reproduce:
1) Open the attached project.
2) In the menu bar, press Tools > Copy To All Scenes.
- An editor extension window shows up.
3) In the Project tab, go to the Assets folder.
4) Drag a prefab called 'GameObject' to the 'Prefab' field of the Editor extension.
5) In the Editor extension window, press 'Copy this prefab to all scenes'.
- The prefab is copied to all the scenes successfully.
- In the 'Prefab' field of the Editor extension, a 'Type Mismatch' error message is displayed.
- Attempts to drag the prefab to the 'Prefab' field are ignored, unless the Editor extension is closed and reopened.