Worked on Xcode 10. Now in the beta I can't build I keep getting this error:
a "WatchKit" is not available when building for iOS Simulator. Consider using
#if !os(iOS)
to conditionally import this framework.
I had the same issue for one swift file in the WatchKit Extension. It turned out that it was a member of both the iOS app and the WatchKit Extension. I unticked the iOS app in the target membership section for the file so that it only belongs to the WatchKit Extension target. Now the project builds successfully.