Facebook和Firebase的身份验证错误

Question

我的问题是，如果我继续或不跳过线程1的错误，则在退出时单击取消按钮时：致命错误：在展开一个可选值时意外发现nil

该错误出现在let凭据= FacebookAuthProvider.credential中（withAccessToken：FBSDKAccessToken.current（）。TokenString）

@IBOutlet weak var butt: FBSDKLoginButton!

in viewDidLoad
butt.delegate = self

in method
    func loginButton(_ loginButton: FBSDKLoginButton!, didCompleteWith result: FBSDKLoginManagerLoginResult!, error: Error!) {
        if error == nil {
            let credential = FacebookAuthProvider.credential(withAccessToken: FBSDKAccessToken.current().tokenString)
            Auth.auth().signInAndRetrieveData(with: credential) { (authResult, error) in
                if let error = error {
                    let alert = UIAlertController(title: "Error", message: error.localizedDescription, preferredStyle: .alert)
                    let accion = UIAlertAction(title: "Ok", style: .default, handler: nil)
                    alert.addAction(accion)
                    self.present(alert,animated: true)
                    return
                }
                print(credential)
                self.performSegue(withIdentifier: "fromLoginToDuringTheTrip", sender: self)
            }
        }