I got this error even with the new version of Beta6 that was release hours after the bad one got pulled.
There are some significant changes in Beta 6 and looks like Xcode is getting confused. A full clean didn’t work either.
I’ve solved this and other similarly illegible errors by deleting the contents of the Derived folder. You can find where that folder is located by going to Preferences > Locations.
The default path is: /Users/[your username]/Library/Developer/Xcode/DerivedData