title: Storyboard scene not displaying in Xcode9
body: <p>Updated to Xcode 9 and now some of my storyboard scenes are blocked out as shown below, and won't display in preview.  I can select the controls but obviously editing is severely impaired.   Based on related posts I tried cleaning the project, deleting derived data, and setting the interface builder document "Opens in" to the earlier version of Xcode, all with restarts.  No results. </p>
  
  <p>I've seen Xcode do this before but the issue has worked itself out quickly.</p>
  
  <p>Any other ideas?</p>
  
  <p><a href="https://i.stack.imgur.com/TMXLj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TMXLj.png" alt="enter image description here"></a></p>