title: sceneView.getSession() returns null
body: <p>I have sceneform sample hellosceneform app with its arFragment and want to add object on custom coordinates. For this, as I understood, I need to have a session to add an Anchor on it then create a Node etc. But when I run <code>arFragment.getArSceneView().getSession()</code> to call <code>createAnchor()</code> on it, I get <code>null</code>. How to get <strong>not null</strong> session?</p>
  
  <p>P.S.  arFragment is standard hellosceneform's ux fragment:
  <code>arFragment = (ArFragment) getSupportFragmentManager().findFragmentById(R.id.ux_fragment)</code></p>