On each fresh import this same bug raises it's ugly head. This bug lives in some of the optional sample content.
_"Error while saving Prefab: 'Assets/AC_Files/GraphicOptions/GraphicOptionsUI.prefab'. You are trying to save a Prefab with a missing script. This is not allowed. Please change the script or remove it from the GameObject 'btnApply'."
_
It's an easy fix, there is a broken reference within the GraphicOptionsUI.prefab, on an object named, "btnApply". An applied script appears to have lost it's field reference. Remove the script reference from the prefab and the issue is resolved. I thought I'd bring it up because I've tested this against (3) fresh project imports and the error has occurred in each instance.
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Comments
An unfortunate leftover from the template's earlier iteration as a downloadable package.
Apologies for these sample content teething issues - I'll see this gets addressed.