title: Trying to access an .stl file using MDLAssets from iCloud but .startAccessingSecurityScopedResource() is allows false
body: <p>Im trying to load a modal into a ARKit Scene which works on top of SceneKit but I'm trying to load a .stl file from iCloud using a UIDocumentPickerViewController which allowed me to save the file Urls into core data for later use. The CoreData Entity is displayed on a TableView for Selection, when I tap one of the items it should load into the scene but <code>url.startAccessingSecurityScopedResource()</code> seems to always be false or without the if statement containing <code>.startAccessingSecurityScopedResource()</code> it crashes at <code>asset.Object(at: 0)</code></p>
  
  <p>here's the func code:</p>
  
  <pre><code>func loadModalToScene(url: URL) {
  
      print("Can Open: ", MDLAsset.canImportFileExtension(url.absoluteString))
      if (url.startAccessingSecurityScopedResource()) {
  
          //Loads 3D Objects into Memory via Model / IO
          let asset = MDLAsset(url: url)
          print("MDL Asset: ", asset)
  
          // Turns the MDLAsset into a MDLObject
          let object = asset.object(at: 0)
          print("Object Asset: ", object)
  
          // Turns the MDLObject into a SCNNode
          let node = SCNNode(mdlObject: object)
          print("SCN Node Asset: ", node)
  
          url.stopAccessingSecurityScopedResource()
      }
  }
  </code></pre>
  
  <p>the function isn't feature complete but I'm trying first to load it into memory.</p>