[iOS] Warning: UIAlertController is already presenting

UIAlert 彈 2次會出問題，解法：

if presentedViewController == nil {
     self.presentViewController(alertController, animated: true, completion: nil)
} else{
      self.dismissViewControllerAnimated(false) { () -> Void in
           self.presentViewController(alertController, animated: true, completion: nil)
      }
}