title: navigation.xml : activities missing, preview and linking unavailable
body: <p>I've just learned about the whole navigation feature in Android, how you can draw a graph and link fragments or activities in order to visualize the flow and pass intents parameters safely through a navigation.xml file.</p>
  
  <p>I have 3 different issues right from the start :</p>
  
  <ol>
  <li>Not all activities can be added to the graph through the editor (I'm aware I can write the XML in the text tab but I believe there's some kind of issue)</li>
  <li>All previews are missing (not a big deal again but still intriguing)</li>
  <li>I cannot link the activities</li>
  </ol>
  
  <p>Do you have any clues of what could've caused this ?</p>