title: how to use 3d models in scenekit?
body: <p>So i am starting to learn scenekit and i have some models i want to try on the iphone simulator so i just changed </p>
  
  <pre><code>let scene = SCNScene(named: "art.scassets/ship.scn")!
  </code></pre>
  
  <p>which works fine in the scenekit default file to</p>
  
  <pre><code>let scene = SCNScene(named: "art.scassets/battleship.scn")!
  </code></pre>
  
  <p>which is a 3d model of a battleship but when i run it it gives me an error near a line of code:</p>
  
  <pre><code>let ship = scene.rootNode.childNodeWithName("ship", recursively: true)!
  </code></pre>
  
  <p>The error says: Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP,subcode=0x0)
  And: fatal error: unexpectedly found nil while unwrapping an Optional value (lldb)</p>
  
  <p>I'm sorry but it is very hard to explain, but if anyone understood what i said please tell me how to import 3d models to scenekit, i haven't found any tutorial that shows how to do that custom 3d models to scenekit</p>