This may just need to be closed, we may not be able to control this at such a level. This really comes up more than you would guess. I think almost everyone has made a widget, wanted to call Editor functionality, then gone "Ah ha! I can reparent it to EditorUtilityWidget" then later banged their head against the wall as to why they don't see editor only functions. I've helped two people just this week with this and probably 4 or 5 others in the last couple of months.
Either not showing the editor classes in the reparent dialog or actually converting it to an Editor Utiltiy Widget would be fixes but they both seem rough.
Create a regular Widget
Change its parent to EditorUtilityWidget
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-92993 in the post.