title: Unknown Window class (null) in Interface Builder file, creating generic Window instead
body: <p>My app uses storyboards and the windows <strong>have to use</strong> a custom class, <code>MyWindow</code> for example, <strong>or</strong> at runtime/launching the app, there is an error message saying:</p>
  
  <blockquote>
    <p>Unknown Window class (null) in Interface Builder file, creating
    generic Window instead</p>
  </blockquote>
  
  <p>I just noticed this after upgrading to Xcode 9.1, but I'm not sure if it was there before.</p>
  
  <p>A similar question is here: <a href="https://stackoverflow.com/questions/24924966/xcode-6-strange-bug-unknown-class-in-interface-builder-file">Xcode 6 Strange Bug: Unknown class in Interface Builder file</a></p>
  
  <h2>Update:</h2>
  
  <p>My other project doesn't have this issue. I will investigate it later.</p>
  
  <h2>Update2:</h2>
  
  <p>In both projects, a window is owned by a custom window controller. I checked the storyboard xml but didn't find anything odd.</p>