I fixed this error message by a simple execution of XCode> Product> Clean Build Folder I don't know how or why the error even incurred, or what the nature of the problem is, though I upgraded from a beta to an RC so stuff like this isn't unexpected

This Developer tool should be automatically and manually tested before a new release! Not after the fact. Don't treat web developers as a second class citizen. Also, the advice isn't workable. There's no apparent place for Preview 125. Preview 126 can't be used, because it requires OS X 12 beta. A web developer should not upgrade their work machine to some beta operating system just because Apple QA is lacking.