Normal - UEFN - Sky texture can be changed but not reverted

I accidentally dragged a texture into the sky making it change, however Undo did not revert the action and now I am stuck with that sky material.
To repro import the attached file and drag it to the sky.
sprite_rock0

Also I can’t get this message off now too:
UnrealEditorFortnite-Win64-Shipping_LubwwdUu9h