title: swift fatal error: unexpectedly found nil while unwrapping an Optional value in adMob
body: <p>I try to ad adMob in my app. 
  After release of my app, adMob not worked.
  I tried to fix it, but now i have the fatal error...
  What is problem in line "bannerView.adUnitID = ...."?
  Thanks...</p>
  
  <pre><code>class ViewController: UIViewController, GADBannerViewDelegate {
  
  @IBOutlet weak var bannerView: GADBannerView!
  
  override func viewDidLoad() {
      super.viewDidLoad()
  
      //ADMOB
      self.bannerView = GADBannerView()
      bannerView.adUnitID = "ca-app-pub-...." // THIS LINE IS FATAL ERROR
      bannerView.delegate = self
      self.bannerView.rootViewController = self
  
      var request: GADRequest = GADRequest()
      //request.testDevices = [""]
      self.bannerView.loadRequest(request)
      }
  </code></pre>