title: How to handle back button when at the starting destination of the navigation component
body: <p>I've started working with the new navigation component and I'm really digging it! I do have one issue though - How am I supposed to handle the back button when I'm at the starting destination of the graph?</p>
  
  <p>This is the code I'm using now:</p>
  
  <pre><code>findNavController(this, R.id.my_nav_host_fragment)
                  .navigateUp()
  </code></pre>
  
  <p>When I'm anywhere on my graph, it's working great, it send me back, but when I'm at the start of it - the app crashes since the backstack is empty.</p>
  
  <p>This all makes sense to me, I'm just not sure how to handle it.</p>
  
  <p>While I can check if the current fragment's ID is the same as the one that I know to be the root of the graph, I'm looking for a more elegant solution like some bool flag of wether or not the current location in the graph is the starting location or not.</p>
  
  <p>Ideas?</p>