title: Unexpectedly found nil while unwrapping an Optional value - boxNode.addChildNode(importedBox!)
body: <p>I have some code that runs a for loop to create 2 objects and stuffs them into the boxArray then adds them to the scene. The first time through the code works fine to create one box but during the addBox(num: Int) function to create the second box it crashes with the error in the subject line. Its this line that is responsible "boxNode.addChildNode(importedBox!)" but Im at a loss as to why since it works the first time. </p>
  
  <pre><code>    importedSceneBox = SCNScene(named: "art.scnassets/boxObject.dae")
  
      for var i = 0; i &lt;= 1; ++i {
  
          let boxNode = self.addBox(i)
          boxArray.append(boxNode)
          theScene.rootNode.addChildNode(boxArray[i])
      }
  
  func addBox(num: Int) -&gt; SCNNode{
  
      let boxNode = SCNNode()
      let importedBox = importedSceneBox.rootNode.childNodeWithName("pCube4", recursively: false)
      importedBox?.scale = SCNVector3Make(70, 70, 70)
      boxNode.addChildNode(importedBox!)
      boxNode.position = SCNVector3Make(5, 100, 3)
      let collisionBox = SCNBox(width: 5.0, height: 5.0, length: 5.0, chamferRadius: 0)
      boxNode.physicsBody?.physicsShape = SCNPhysicsShape(geometry: collisionBox, options: nil)
      boxNode.physicsBody = SCNPhysicsBody.dynamicBody()
      boxNode.physicsBody?.mass = 1
      boxNode.physicsBody?.restitution = 0.8
      boxNode.physicsBody?.damping = 0.5
      boxNode.name = "dice" + String(num)
      boxNode.physicsBody?.allowsResting = true
  
      return boxNode
  }
  </code></pre>