'Build input file cannot be found' Swift 4.2, Xcode 10.0

Question

I am getting the following error when building in Xcode 10.0 with swift 4.2:

Answer 1

If all else fails:

1. In Xcode. Remove references of files in question from your project.
2. Open your project.pbxproj file as a file in Xcode. This is in .xcodeproj
3. Delete all lines containing the name of said files. Save.
4. Back in Xcode. Re-add files to your project in Xcode.