Storyboard doesn't contain a controller with identifier 'MainWindow'

Question

I am trying to instantiate an NSWindowController from a storyboard in my OS X app:

NSWindowController *mainWindowController = [[NSStoryboard story

Answer 1

Confirmed in 11.3. This is such a time-waster, together with this one:

How do I open another window in macOS in Swift with Cocoa

Answer 2

It was definitely a bug with Xcode. Restarting Xcode solved the problem.