let alert = UIAlertController(title: "Thông báo", message: "Vui lòng chọn một cửa hàng", preferredStyle: UIAlertControllerStyle.alert)
alert.addAction(UIAlertAction(title: "Đồng ý", style: .cancel, handler: nil))
// assuming that this is the first time your controller is presented
present(alert, animated: true, completion: nil)
//something bad happened and your application is trying to present the alert again, safe guard against presenting it twice by checking if presentedViewController is nil or not
if presentedViewController == nil {
present(alert, animated: true, completion: nil)
}