title: ARSession runs but without configuration
body: <p>When I run my <code>ARSession</code> with <code>ARWorldTrackingSessionConfiguration</code> the <code>sceneView.session</code> property doesn't seem to hold on to the configuration. This is my code</p>
  
  <pre><code>override func viewWillAppear(_ animated: Bool) {
      super.viewWillAppear(animated)
  
      // Create a session configuration
      let configuration = ARWorldTrackingSessionConfiguration()
  
      configuration.planeDetection = .horizontal
  
      // Run the view's session
      sceneView.session.run(configuration)
  }
  </code></pre>
  
  <p>I put a breakpoint on <code>sceneView.session.run(configuration)</code> and it gave me the following</p>
  
  <pre><code>po sceneView.session
  &lt;ARSession: 0x1c83c8520 currentFrame=0x0 timestamp=0.000000 configuration=(null)&gt;
  </code></pre>
  
  <p>Note: <code>configuration=(null)</code></p>
  
  <p>I'm later trying to access <code>sceneView.session.currentFrame</code> but it gives me nil, so I assume the above to be the issue.</p>
  
  <p>I'm running it on an iPad Pro 2017, it does support <code>ARWorldTrackingSessionConfiguration</code>, and I've also tried only using <code>ARSessionConfiguration</code> but that gives me the same thing.</p>
  
  <p>Has anyone else experienced this? It kind of looks like an ARKit bug right now, since my code is taken straight from the docs. Any ideas?</p>