Swift：应用程序内的电子邮件冻结几秒钟并抛出异常

Question

我正在尝试将应用内电子邮件功能添加到我的应用中。我在网上找到了一些代码，但是当我运行应用程序时，它只会冻结几秒钟并抛出异常。以下是我使用的代码

    @IBAction func sendMail(sender: AnyObject) {
    let mailComposeViewController = configuredMailComposeViewController()
    if MFMailComposeViewController.canSendMail() {
        self.presentViewController(mailComposeViewController, animated: true, completion: nil)
    } else {
        self.showSendMailErrorAlert()
    }
}

func configuredMailComposeViewController() -> MFMailComposeViewController {
    let mailComposerVC = MFMailComposeViewController()
    mailComposerVC.mailComposeDelegate = self

    mailComposerVC.setToRecipients(["someone@somewhere.com"])
    mailComposerVC.setSubject("Sending you an in-app e-mail...")
    mailComposerVC.setMessageBody("Body", isHTML: false)

    return mailComposerVC
}

func showSendMailErrorAlert() {
    let sendMailErrorAlert = UIAlertView(title: "Could Not Send Email", message: "Your device could not send e-mail.  Please check e-mail configuration and try again.", delegate: self, cancelButtonTitle: "OK")
    sendMailErrorAlert.show()
}

// MFMailComposeViewControllerDelegate Method
func mailComposeController(controller: MFMailComposeViewController!, didFinishWithResult result: MFMailComposeResult, error: NSError!) {
    controller.dismissViewControllerAnimated(true, completion: nil)
}

例外是：

*** Assertion failure in -[UIKeyboardTaskQueue waitUntilAllTasksAreFinished], /SourceCache/UIKit_Sim/UIKit-3318.16.14/Keyboard/UIKeyboardTaskQueue.m:374
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[UIKeyboardTaskQueue waitUntilAllTasksAreFinished] may only be called from the main thread.'

有些帖子建议我们需要在每次使用后循环MFMailComposeViewController。但我真的不知道如何实现它。