ios - fatal error : unexpectedly found nil while unwrapping an Optional value on performing segue

Question

我正在使用 facebook SDK 登录，一切都已正确设置。当我第一次授权我的应用程序时，segue 将执行到我的下一个 View Controller 。但是当我注销并尝试再次登录时，segue 出现错误

fatal error :在解包可选值时意外发现 nil

这是我的 appDelegate 文件中的代码

func application(application: UIApplication, openURL url: NSURL, sourceApplication: String?, annotation: AnyObject) -> Bool{

    let result = FBSDKApplicationDelegate.sharedInstance().application(application, openURL: url, sourceApplication: sourceApplication, annotation: annotation)

    if result {

        let token = FBSDKAccessToken.currentAccessToken()
        let fieldsMapping = [
            "id" : "facebookId",
            "name" : "name",
            "email": "email"
        ]

        backendless.userService.loginWithFacebookSDK(
            token,
            fieldsMapping: fieldsMapping,
            response: { (user: BackendlessUser!) -> Void in

                print("user: \(user)")
                self.backendless.userService.setStayLoggedIn(true)
                loadUser()
                // Access the storyboard and fetch an instance of the view controller
                let storyboard = UIStoryboard(name: "Main", bundle: nil)
                let viewController: MainScreenViewController = storyboard.instantiateViewControllerWithIdentifier("loginViewController") as! MainScreenViewController

                let rootViewController = self.window!.rootViewController! as UIViewController
                print(rootViewController)
                rootViewController.presentViewController(viewController, animated: true, completion: nil)


            },
            error: { (fault: Fault!) -> Void in
                print("Server reported an error: \(fault)")
        })
    }

    return result
}

错误发生在rootViewController.presentViewController(viewController,animated:true,completion:nil)

我尝试在 viewDidAppear() 中的 viewController 中执行 segue，并且还尝试了 PerformSegueWithIdentifier() 但错误仍然存​​在。如果我终止我的应用程序并重试，segue 工作正常。请建议我问题出在哪里。提前致谢。

Answer 1

我认为没有 rootViewController 所以你需要像下面这样设置

 func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool {

 var rootView: MyRootViewController = MyRootViewController()

 if let window = self.window{
        window.rootViewController = rootView
 }
 return true
}

然后在后记中使用它。