title: ios popToRootViewControllerAnimated, its animated parameter setting issue
body: <p>See apple doc as below, does it mean we must set parameter animated as NO if we create navigation controller in advance? Such as the case as following. 
  1. Create a navigation controller. 
  2. Push view controllers in navigation controller. 
  3. Then invoke popToRootViewControllerAnimated. </p>
  
  <p>Ask this question dues to I have experienced many strange issues (including crash) regarding animated setting in my project. Any discussion will be appreciated. </p>
  
  <pre><code>popToRootViewControllerAnimated:
  Parameters
  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.
  </code></pre>