Swift performSegueWithIdentifier 不起作用

我试图在用户成功登录到他们的帐户后切换视图控制器，但它无法正常工作.我不能直接使用segue，因为如果单击登录按钮，无论信息是否正确，它都会转到该视图控制器.我try 了我所知道的一切，但没有成功.这就是我正在try 的代码.

   @IBAction func loginTapped(sender: AnyObject) {

    let username = usernameField.text
    let password = passwordField.text

    if username.isEmpty || password.isEmpty {
        var emptyFieldsError:UIAlertView = UIAlertView(title: "Please try again", message: "Please fill in all the fields we can get you logged in to your account.", delegate: self, cancelButtonTitle: "Try again")
        emptyFieldsError.show()
    }

    PFUser.logInWithUsernameInBackground(username, password:password) {
        (user: PFUser?, error: NSError?) -> Void in
        if user != nil {
            self.performSegueWithIdentifier("Klikur", sender: self)
        } else {
            if let errorString = error!.userInfo?["error"] as? String {
                self.errorMessage = errorString
            }

            self.alertView("Please try again", message: "The username password combiation you have given us does not match our records, please try again.", buttonName: "Try again")
        }
    }

}

我将情节提要ID设置为"测试"，当输入正确的信息时，它不会切换视图控制器.有人能帮我解决问题吗？