Ios 从FIREBASE_REF块内快速关闭vc控制器

我只是在寻找帮助，关闭createuser vc并返回到以前的初始vc（如果firebase中已经存在用户）

我目前有：

// Attempt to create a new user.  (authData = dictionary).
        FIREBASE_REF.createUser(email, password: password, withValueCompletionBlock: { (error, authData) in

// If there was no error then
            if error == nil {
// Authorise the new user into the application.  (authData = object).
                FIREBASE_REF.authUser(email, password: password, withCompletionBlock: { ( error, authData) in

                    if error == nil {
// Store the user id into the device.
                        NSUserDefaults.standardUserDefaults().setValue(authData.uid, forKey: "uid")
// Stop the activity indicator.
                        self.activityInd.stopAnimating()
// Close the create user screen.
                        self.dismissViewControllerAnimated(true, completion: nil)
                    }
                    else
                    {
// Stop the activity indicator.
                        self.activityInd.stopAnimating()

                        print(error)

                   }
                })
            }
            else

// USER ACCOUNT ALREADY EXISTS \\
            {
// Stop the activity indicator.
                self.activityInd.stopAnimating()

// Display an error message.
                let alertController = UIAlertController(title: "USER DUPLICATION ERROR", message:
                    "user account already exists, login as normal", preferredStyle: UIAlertControllerStyle.Alert)
                alertController.addAction(UIAlertAction(title: "Dismiss", style: UIAlertActionStyle.Default,handler: nil))
                self.presentViewController(alertController, animated: true, completion: nil)

// Close the create user screen.
                self.dismissViewControllerAnimated(true, completion: nil)

            }
      })

当用户创建一个新帐户时，self.dismissViewController工作正常，返回初始vc，然后加载主页。但是，当已有用户时，self.dismissViewController不会执行任何操作。如果有人能对此有所了解，我们将不胜感激

此外，如果有人能建议如何确保警报消息在用户选择关闭vc之前一直显示在屏幕上，这也会很有帮助

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谢谢

问题解决了，如果有人感兴趣，以下是我如何解决这个问题的：

// USER ACCOUNT ALREADY EXISTS \\
            {
// Stop the activity indicator.
                self.activityInd.stopAnimating()

// Display an error message.
                let refreshAlert = UIAlertController(title: "USER DUPLICATION ERROR", message: "user account already exists, login as normal.", preferredStyle: UIAlertControllerStyle.Alert)

                refreshAlert.addAction(UIAlertAction(title: "Dismiss", style: .Default, handler: { (action: UIAlertAction!) in
// Close the create user screen.
                    self.dismissViewControllerAnimated(true, completion: nil)
                }))

                self.presentViewController(refreshAlert, animated: true, completion: nil)
            }
      })

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我已经缩小了范围，当我注释掉警报消息时，它会关闭屏幕并按其应该的方式返回。显然，警报消息导致了问题，如果有人能告诉我为什么会导致问题，我将不胜感激。我怀疑一旦调用了警报，它就不再在代码中继续。