UIColor(named:) always returns nil on iOS 11.0-11.2

Edit:

This was a bug in Xcode and its fixed in the new GM Seed 2 (11A420a).

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I encountered this issue as well. I was using Xcode 11 beta 6, with Command Line Tools 11.0.

I solved it by switching to Xcode 10.2.1 and Command Line Tools 10.2.1, cleaning & building again.