Admob banner Thread 1 error

Question

I would like to show an adMob banner. I think I've configured adMob correctly but when I launch the app it crashes giving the famous error: Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value . I don't know what I am doing wrong. The error is on the line banner1.adUnitID = "ca-app-pub-1314395885924826/98794********" ViewController.swift:

import UIKit
import Foundation
import MessageUI
import Firebase
import GoogleMobileAds

class ViewController: UIViewController, MFMailComposeViewControllerDelegate {

    @IBOutlet weak var banner1: GADBannerView!

    override func viewDidLoad() {

        banner1.adUnitID = "ca-app-pub-1314395885924826/98794********"    //Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value
        banner1.rootViewController = self
        banner1.load(GADRequest())
        // Do any additional setup after loading the view, typically from a nib.
    }

Answer 1

Make sure banner1 is referenced correctly from your storyboard to the ViewController. Also make sure the class of banner1 in storyboard is assigned to GADBannerView.