MFMailComposeViewController not dismiss with Cancel按钮

Question

我有一个类，它是带有函数的NSObject的子类，可以显示一个MFMailComposeViewController。代码如下：

MFMailComposeViewController *mailController = [[MFMailComposeViewController alloc] init];

mailController.mailComposeDelegate = self;
[mailController setSubject:@"Sample Subject"];
[mailController setMessageBody:@"Here is some main text in the email!" isHTML:NO];
[mailController setToRecipients:@[self.email]];

UITabBarController *tabbarController = (UITabBarController *)[UIApplication sharedApplication].keyWindow.rootViewController;
UINavigationController *navigationController = tabbarController.selectedViewController;
[navigationController.topViewController presentViewController:mailController animated:YES completion:NULL];

这段代码一切都运行得很好。问题是当我想取消MFMailComposeViewController的时候。有时候我会崩溃，有时候什么都不会发生。我已经实现了委托函数：

- (void)mailComposeController:(MFMailComposeViewController *)controller didFinishWithResult:(MFMailComposeResult)result error:(NSError *)error {
    UITabBarController *tabbarController = (UITabBarController *)[UIApplication sharedApplication].keyWindow.rootViewController;
    UINavigationController *navigationController = tabbarController.selectedViewController;
    [navigationController.topViewController dismissViewControllerAnimated:YES completion:nil];
}

在那之后，我试着在ViewController上直接显示并关闭它，一切都在工作。甚至是取消按钮。

我不知道为什么它在我的ViewController类中有效，但在我的NSObject子类中无效。

当我在日志中看到崩溃时：

-[MFMailComposeInternalViewController _notifyCompositionDidFinish]