Интеграция AdMob: Ошибка: Тема 1: Неустранимая ошибка:

Я пытаюсь добавить admob в свое приложение, но получаю сообщение об ошибке.

Тема 1: Неустранимая ошибка: неожиданно найдено nil при развертывании необязательного значения

Я следил за сайтами developer.google.com и stackoverflow.com, но не могу это исправить.

Вот мой код:

импортировать UIKit импортировать GoogleMobileAds

класс ViewController: UIViewController, GADBannerViewDelegate {

//Place your instance variables here
let allQuestions = QuestionBank()
var pickedAnswer : Bool = false
var questionNumber : Int = 0
var score : Int = 0
var bannerView: GADBannerView!
var banner: GADBannerView!


@IBOutlet weak var questionLabel: UILabel!
@IBOutlet weak var scoreLabel: UILabel!
@IBOutlet weak var GoogleBannerView: GADBannerView!

@IBOutlet weak var progressLabel: UILabel!

override func viewDidLoad() {
    super.viewDidLoad()

    banner = GADBannerView(frame: self.view.frame)
    GADMobileAds.configure(withApplicationID: "ca-app-pub-3940256099942544/2934735716")
    bannerView.adUnitID = "ca-app-pub-3940256099942544/2934735716"       ***//Crashes Here***
    bannerView.rootViewController = self
    bannerView.load(GADRequest())
    bannerView.delegate = self

    /// Tells the delegate an ad request loaded an ad.
    func adViewDidReceiveAd(_ bannerView: GADBannerView) {
        print("adViewDidReceiveAd")
    }

    /// Tells the delegate an ad request failed.
    func adView(_ bannerView: GADBannerView,
                didFailToReceiveAdWithError error: GADRequestError) {
        print("adView:didFailToReceiveAdWithError: \(error.localizedDescription)")
    }

    /// Tells the delegate that a full-screen view will be presented in response
    /// to the user clicking on an ad.
    func adViewWillPresentScreen(_ bannerView: GADBannerView) {
        print("adViewWillPresentScreen")
    }

    /// Tells the delegate that the full-screen view will be dismissed.
    func adViewWillDismissScreen(_ bannerView: GADBannerView) {
        print("adViewWillDismissScreen")
    }

    /// Tells the delegate that the full-screen view has been dismissed.
    func adViewDidDismissScreen(_ bannerView: GADBannerView) {
        print("adViewDidDismissScreen")
    }

    /// Tells the delegate that a user click will open another app (such as
    /// the App Store), backgrounding the current app.
    func adViewWillLeaveApplication(_ bannerView: GADBannerView) {
        print("adViewWillLeaveApplication")
    }

    bannerView = GADBannerView(adSize: kGADAdSizeBanner)

    addBannerViewToView(bannerView)

    nextQuestion()
}

func addBannerViewToView(_ bannerView: GADBannerView) {
    bannerView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(bannerView)
    view.addConstraints(
        [NSLayoutConstraint(item: bannerView,
                            attribute: .bottom,
                            relatedBy: .equal,
                            toItem: bottomLayoutGuide,
                            attribute: .top,
                            multiplier: 1,
                            constant: 0),
         NSLayoutConstraint(item: bannerView,
                            attribute: .centerX,
                            relatedBy: .equal,
                            toItem: view,
                            attribute: .centerX,
                            multiplier: 1,
                            constant: 0)
        ])
}

Спасибо всем, кто может помочь!!