title: Crash when invoking popToRootViewControllerAnimated:NO, set animate to YES work no crash, strange.
body: <p>My app is based on UINavigationViewController, there are 3 ViewController in the stack, named A, B, C,  A is root view controller, now in ViewController C do the following operation, app will crash 100%. </p>
  
  <pre><code>- (void) delayPopAction {
      [[self navigationController] popToRootViewControllerAnimated:NO];
  }
  
  - (void)searchTap:(id)sender{
      [self performSelector:@selector(delayPopAction) withObject:nil afterDelay:3.0];
  }
  </code></pre>
  
  <p>Trace by zombie and find the crash dues to C has been destroyed but there is still event available sent to him. More strange, if I change animated parameter from NO to YES, no crash happen, maybe dues to C's life circle has been enlarged. </p>
  
  <p>See API doc. How does NO mean? I don't understand it clearly.<br>
  animated
  Set this value to YES to animate the transition. Pass NO if you are setting up a navigation controller before its view is displayed.  </p>
  
  <p>Any idea or suggestion will be appreciated, thanks in advance. </p>