In Xcode 10, I'm having this issue where previously working extensions (from the store and some of mine), that have been working fine for a long time, are suddenly disappearing from the Editor menu and System Prefs - > Extensions.
Restarting Xcode doesn't help.
Restarting Mojave doesn't help.
The only thing that helps is downloading a fresh install of Xcode (clearly an unworkable solution).
Is anybody else experiencing this or has managed to resolve it?
This fixed my similar issue:
Actually after step 1, "System Prefs - > Extensions" back to normal, but I just want the default app name back.
I have the same problem.
However you don't have to re-download Xcode to get the extensions back. Just drag Xcode from the Applications folder to somewhere else. Desktop is fine. Then drag it back.
The act of adding Xcode to the Applications folder (again) triggers whatever it is in OS X that reads the extensions.