shouldPerformSeguedでalertをだす

override func shouldPerformSegue(withIdentifier identifier: String, sender: Any?) -> Bool {
        if identifier == "showResult" {
            guard self.sales.text != "" else{
                let alertController = UIAlertController(title: "Error", message: "Please enter your name", preferredStyle: .alert)
                let defaultAction = UIAlertAction(title:"OK", style: .default, handler: nil)
                alertController.addAction(defaultAction)
                self.present(alertController, animated: true, completion: nil)
                return false
            }
            return true
        }
        return true
    }

初期

text fieldがnilの時