title: ActivityViewController very occasionally not appearing
body: <p>In an app with a large number of users, a tiny percentage are seeing a problem where the activity view for sharing videos doesn't appear at all (no error or hang, just nothing happens). Users with this problem tend to have it consistently, until they do something seemingly unconnected like update to the latest version of iOS (even though the same issue has been seen in the latest version of iOS as well). I've never been able to reproduce the issue, so not sure how to debug.</p>
  
  <p>Is there any reason at all in the following code that the activity view might not appear?</p>
  
  <pre><code>func main(closure:@escaping ()-&gt;()) {
      DispatchQueue.main.async {
          closure()
      }
  }
  
  ...
  
  let objectsToShare = [videoUrl]
  
  let activityViewController = UIActivityViewController(activityItems: objectsToShare, applicationActivities: nil)
  
  activityViewController.setValue("Video", forKey: "subject")
  
  activityViewController.excludedActivityTypes = [
      UIActivityType(rawValue: "com.apple.reminders.RemindersEditorExtension"),
      UIActivityType(rawValue: "com.apple.mobilenotes.SharingExtension"),
      UIActivityType.addToReadingList,
      UIActivityType.assignToContact,
      UIActivityType.openInIBooks,
      UIActivityType.print
  ]
  
  main {
  
      activityViewController.modalPresentationStyle = .popover
      activityViewController.popoverPresentationController!.sourceView = masterViewController.view
      activityViewController.popoverPresentationController!.sourceRect = self.shareImage.convert(self.shareImage.bounds, to: masterViewController.view)
  
      activityViewController.completionWithItemsHandler = { activityType, _, _, _ in
          //share success
      }
  
      masterViewController.present(activityViewController, animated: true, completion:nil)
  
  }
  </code></pre>