title: Can't feed a MDLMesh container with 3D model as SCNGeometry
body: <p>I'm creating AR app (Xcode 10.1, Swift 4.2.1). </p>
  
  <p>I'd like to load <code>USDZ</code> 3D object into an <strong>empty</strong> SceneKit's scene and then process it as <code>MDL</code> mesh.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>import ARKit
  import SceneKit.ModelIO
  
  let scene = SCNScene(named: "art.scnassets/emptyScene.scn")!
  
  if let filePath = Bundle.main.path(forResource: "Helicopter", 
                                          ofType: "usdz", 
                                     inDirectory: "art.scnassets") {
  
      let refURL = URL(fileURLWithPath: filePath)
      let refNode = SCNReferenceNode(url: refURL)
      refNode?.load()
      scene.rootNode.addChildNode(refNode!)
  }
  
  let helicopterGeo = refNode!.geometry
  
  let mdlMesh = MDLMesh(scnGeometry: helicopterGeo!)      // ERROR APPEARS HERE
  try! mdlMesh.makeVerticesUniqueAndReturnError()
  let flattenedGeometry = SCNGeometry(mdlMesh: mdlMesh)
  let flattenedNode = SCNNode(geometry: flattenedGeometry)
  scene.rootNode.addChildNode(flattenedNode)
  </code></pre>
  
  <p><br></p>
  
  <blockquote>
    <p>But compiler gives me an error: </p>
  </blockquote>
  
  <pre><code>"Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value"
  </code></pre>
  
  <p><strong><em>The question is</em></strong>: what approach should I use to assign a <strong><code>"Helicopter.usdz"</code></strong> geometry to a <strong><code>helicopterGeo</code></strong> constant?</p>
  
  <p>Help me find a workaround, please! </p>
  
  <p>You can download <code>USDZ</code> file for testing <a href="https://developer.apple.com/arkit/gallery/" rel="nofollow noreferrer">HERE</a>.</p>