Well, that’s annoying. App Store updated my Xcode to v10.0. Even though I had renamed my old one to Xcode9.4.1 it got deleted and isn’t even in the Trash. Will have to download it again.
This happens if you use the Mac App Store. You can download individual Xcodes at developer.apple.com/download/more
I rename my Xcode from Xcode.app to Xcode941.app, Xcode10.app, etc. Works great.
Don’t forget to install command line tools as Xcode 9.4.1
Xcode - Preferences - Locations