Failed to load CustomEditor inspected type UnityEditor.CustomEditorAttributes:FindCustomEditorType(Object)

Failed to load CustomEditor inspected type UnityEditor.CustomEditorAttributes:FindCustomEditorType(Object)

I decided to take a break from my game making so ran my project one last time without error then shut it down. But when I opened it an hour later and tried to run it I got that error. I restarted it and then error went away until I added a box collider then it came back.

What does it mean?

cheers.

Solved.

Re-importing all the standard assets seemed to work.