Unknown Window class (null) in Interface Builder file, creating generic Window instead

This warning also occurs when I build and run a brand new empty project in Xcode 9.1.

Replacing the default NSWindow with an empty custom class of NSWindow with no changes does remove the error, but that behaviour makes no sense.

Therefore, I'm filing it as a bug.

I had the same issue after upgrading to Xcode 9.1. To resolve the error you need to replace all instances of NSWindow in storyboards with custom subclasses of NSWindow.

I have the same problem in several projects since the upgrade.

"Unknown Window class (null) in Interface Builder file, creating generic Window instead"

This happens in all Cocoa projects it seems at the moment. Few of them will run properly. However I have no custom window controllers. Replacing the default window controller in Storyboard with a named subclass doesn't change the situation.

Solved: - Creating blank custom window subclass for the default window solves the problem. I was getting thrown by the message appearing after making changes to the project (accidentally deleting my main viewcontroller etc.)