If Xcode is acting to you the way it is to me, you may want to return from automated migration and just do it by hand. I spent too much time this morning and tried to get Xcode to complete migration for those that were extremely trivial changes.
If you go by hand, make sure you change the launcher
Any] to use
UIApplication.LaunchOptionsKey . (Note the period in the name.)
To set the compiler from Swift 4 to Swift 4.2, open Goals> Build Settings. Navigate to Swift Language Version and use the popup window to select the correct compiler:
For some projects (especially small sample code) this alone may be sufficient to get you back and work.