title: The following code works when I manually pass the value but fails when I unwrap the value
body: <p>I just started learning Swift as my first coding language. My current challenge is trying to automate the transitions from a current level setup as  LevelOne.sks to another level also created in Xcode level editor as LevelTwo.sks. What I'm attempting to do is trigger a transition to the next level with the following set of code.</p>
  
  <p>In my base scene I have this function to send the player to the next level </p>
  
  <pre><code>private func goToNextLevel(nextLevel: String) {
  
      //When hard coding the arguments as... 
      //loadScene(withIdentifier: .levelTwo)
      //The level two scene loads and is playable...however,
      //When trying to change the level argument in code
     // by passing a nextLevel variable
     // the optional unwraps nil and crashes the app.
  
      loadScene(withIdentifier: SceneIdentifier(rawValue: nextLevel)!)
  
  }
  </code></pre>
  
  <p>This is then passed to a SceneLoadManager File</p>
  
  <pre><code>  enum SceneIdentifier: String {
      case levelOne = "LevelOne"
      case levelTwo = "LevelTwo"
  //    case levelThree = "LevelThree"
  }
  
  private let sceneSize = CGSize(width: 768, height: 1024)
  
  protocol SceneManager { }
  extension SceneManager where Self: SKScene {
  
      func loadScene(withIdentifier identifier: SceneIdentifier) {
  
  
          let reveal = SKTransition.flipHorizontal(withDuration: 0.5)
          let nextLevel = SKScene(fileNamed: identifier.rawValue)
          nextLevel?.scaleMode = .aspectFill
          self.view?.presentScene(nextLevel!, transition: reveal)
      }
  </code></pre>
  
  <p>I think this has something to do with how I'm trying to set nextLevel. Currently I'm setting this up as follows</p>
  
  <pre><code>let nxtLvl = String?
  nxtLvl = ".levelOne"
  goToNextLevel(nextLevel: nxtLvl)
  </code></pre>
  
  <p>Hopefully you can make sense of what I'm trying to achieve and that I'm at least close to being on the right track here. Any help would be greatly appreciated. Thanks!</p>