按发送按钮时，发送电子邮件应用程序崩溃

Question

我的iOS发送电子邮件应用程序出现问题。我创建了一个联系表单，用户可以键入姓名，电话号码，电子邮件地址等，最后还有一个发送按钮。当按下发送按钮时，应用程序崩溃。我没有收到任何错误消息来说明我的应用崩溃的原因。

这是我下面的代码。我缺少任何可能导致我的应用崩溃的东西吗？

import UIKit
import MessageUI

class SendEmail: UIViewController, MFMailComposeViewControllerDelegate  {
    @IBOutlet var nameField: UITextField!
    @IBOutlet var emailAddressfield: UITextField!
    @IBOutlet var phoneNumberfield: UITextField!
    @IBOutlet var requestfield: UITextField!

    @IBAction func Send(sender: Any) {
        if !MFMailComposeViewController.canSendMail() {
            print("Email Account Not Found")
        } else {
            let mc = MFMailComposeViewController()
            mc.mailComposeDelegate = self

            //let toRecipients = ["st.johnamechurch@att.net"]
            mc.setToRecipients(["st.johnamechurch@att.net"])
            mc.setSubject("Prayer Request: ")
            mc.setMessageBody("Name: \(nameField.text!) \n\nEmail: \(emailAddressfield.text!) \n\nPhoneNumber: \(phoneNumberfield.text!) \n\nPrayerRequest: \(requestfield.text!)", isHTML: false)

            self.present(mc, animated: true, completion: nil)
        }

        func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {
            switch result.rawValue {
            case MFMailComposeResult.cancelled.rawValue:
                print("Cancelled")

            case MFMailComposeResult.failed.rawValue:
                print("failed")

            case MFMailComposeResult.saved.rawValue:
                print("saved")

            case MFMailComposeResult.sent.rawValue:
                print("sent")

            default:
                break
            }

            controller.dismiss(animated: true, completion: nil)
        }

        func dismissKeyboard(_ sender: AnyObject) {
            self.resignFirstResponder()
        }
    }
}