title: UIActivityViewController completion handler executes before the view controller disappears from the screen
body: <p>I am using UIActivityViewController to present sharing options and I want to display another view controller when the UIActivityViewController is dismissed by the user or when the animation that follows "activityDidFinish:(bool)completed" gets over. When I try to present the other controller in the completion handler of the UIActivityViewController, I get the following warning and the second VC does not get displayed at all!</p>
  
  <p><strong>Attempt to present &lt;_UIViewController: 0x1e16f020> on &lt;###> while a presentation is in progress!</strong></p>
  
  <pre><code>UIActivityViewController activityVC = [[UIActivityViewController alloc]initWithActivityItems:selectedAssetsURL applicationActivities:nil];
  
  [activityVC setCompletionHandler:^(NSString *activityType, BOOL completed) {
      NSLog(@"completed");
      //Present another VC
  }];
  </code></pre>
  
  <p>The question is, how to know when the UIActivityViewController disappears from the screen? Even the -ViewDidAppear method of the view controller which presents the UIActivityViewController does not get fired!</p>