title: ARKit - Issue with ARSession
body: <p>I'm trying to create <code>ARSession</code>, using ARKit with Xcode 9 beta (for iOS 11). But it seems not working properly.</p>
  
  <p>Code I've tried is:</p>
  
  <pre><code>override func viewDidLoad() {
  
      super.viewDidLoad()
  
      // configure session
      let configuration = ARWorldTrackingSessionConfiguration()
      configuration.planeDetection = .horizontal
  
      // run session
      sceneView.session.run(configuration)
  }
  </code></pre>
  
  <p>Can anyone help? Code is almost correct according to Apple Documentation.</p>