So I deleted the sky_sphere by accident and saved.
did some reading online and saw another guy had this too. If I open up another project I can see it in the classes search. On the project where I deleted it there is no entry for sky_sphere.
It should still be in engine content, you can try to search/find it in the engine content, by checking the “show engine content”, then searching for it 
Perfect, Found it in there as you mentioned
Thanks