MFMailComposeViewController和MFMessageComposeViewController都会在“取消”按钮上发生崩溃

Question

我创建了一个用于发送邮件和短信的实用程序类。这是完整的代码。

class Utility: NSObject, MFMailComposeViewControllerDelegate, MFMessageComposeViewControllerDelegate
{
    var subject: String!
    var body: String!
    var to: [String]!
    var cc: [String]!
    var viewController: UIViewController!

    // MARK: Email utility

    /// sendEmail : Sends email
    ///
    /// - Parameters:
    ///   - vc: UIViewController from which mail is to be sent
    ///   - subj: Email subject
    ///   - emailBody: Email body
    ///   - toRecipients: To recipients
    ///   - ccRecipients: CC recipients
    func sendEmail(vc: UIViewController, subj: String, emailBody: String, toRecipients: [String], ccRecipients: [String])
    {
        self.subject = subj
        self.body = emailBody
        self.to = toRecipients
        self.cc = ccRecipients
        self.viewController = vc
        let mailComposeViewController = configuredMailComposeViewController()
        if MFMailComposeViewController.canSendMail() {
            self.viewController.present(mailComposeViewController, animated: true, completion: nil)
        } else {
            self.showSendMailErrorAlert()
        }
    }


    /// Show error alert when email could not be sent
    func showSendMailErrorAlert() {
        let alert = UIAlertController(title: "Could Not Send Email", message: "Your device could not send e-mail.  Please check e-mail configuration and try again.", preferredStyle: UIAlertControllerStyle.alert)
        alert.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.default, handler: nil))
        self.viewController.present(alert, animated: true, completion: nil)
    }


    /// configuredMailComposeViewController : Set up mail compose view controller
    ///
    /// - Returns: <#return value description#>
    func configuredMailComposeViewController() -> MFMailComposeViewController {
        let mailComposerVC = MFMailComposeViewController()
        mailComposerVC.mailComposeDelegate = self
        mailComposerVC.setToRecipients(self.to)
        mailComposerVC.setCcRecipients(self.cc)
        mailComposerVC.setSubject(self.subject)
        mailComposerVC.setMessageBody(self.body, isHTML: false)

        return mailComposerVC
    }

    // MARK: MFMailComposeViewControllerDelegate
    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {
        controller.dismiss(animated: true, completion: nil)
    }

    // MARK: END

    // MARK: SMS utility

    func sendSMS(vc: UIViewController, content: String, phoneNumber: String)
    {
        self.viewController = vc
        if (MFMessageComposeViewController.canSendText()) {
            let messageController = MFMessageComposeViewController()
            messageController.body = content
            messageController.recipients = [phoneNumber]
            messageController.messageComposeDelegate = self
            self.viewController.present(messageController, animated: true, completion: nil)
        }
    }

    func messageComposeViewController(_ controller: MFMessageComposeViewController, didFinishWith result: MessageComposeResult) {
        //... handle sms screen actions
        controller.dismiss(animated: true, completion: nil)
    }

    // MARK: END  

两个功能上的取消按钮都会崩溃。事实上，如果我在didFinishWith上为这两个功能设置断点，则不会调用该函数并在此之前出现崩溃。
奇怪的是，如果我在UIViewController类中编写此代码，它可以正常工作。