title: failed assertion `vertexFunction must not be nil - Error when loading a SceneKit scene in iPad iOS 9.0.2
body: <p>I am getting a frustrating error when testing my app on the iPad , at the moment my SCNScene gets loaded, as soon as I see the nodes on the screen. I got no idea what it could be:</p>
  
  <pre><code>/BuildRoot/Library/Caches/com.apple.xbs/Sources/Metal/Metal-54.31/Framework/MTLRenderPipeline.mm:973: failed assertion `vertexFunction must not be nil.
  </code></pre>
  
  <p>I'm not directly using Metal, neither OpenGL, only SceneKit stuff. I don't understand much of them but I heard they are somehow used underneath.
  I have tried to fix it by "forcing" OpenGL, as I did read in some topic that it could help:</p>
  
  <pre><code>scnView = GGBattleScene2(frame: view.frame, options: [SCNPreferredRenderingAPIKey : SCNRenderingAPI.OpenGLES2.rawValue])
  </code></pre>
  
  <p>The scene appeared, but a lot of elements added just got invisible, or not rendered, i don't know. Actually all of them with exception of the support axis I've drawn:</p>
  
  <p><a href="https://i.stack.imgur.com/H8ram.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/H8ram.jpg" alt="axis visible"></a></p>