title: Android Navigation popBackStack
body: <p>I tried use Android Navigation component and have a problem with back stack.</p>
  
  <p>I have Fragment A, B.
  I write:</p>
  
  <pre><code>Navigation.findNavController(view).navigate(R.id.a_to_b)
  </code></pre>
  
  <p>It's ok. But if I want return to A, i call:</p>
  
  <pre><code>Navigation.findNavController(view).popBackStack();
  </code></pre>
  
  <p>Then in fragment B will be calling onViewStateRestored with <strong>null</strong> state
  why?</p>
  
  <p>I want to keep state when calling navigate to Fragment A. </p>