UIViewController dismissModalViewControllerAnimated:导致主窗口消失

我想在iPad应用程序中呈现一个这样的模式邮件对话:

MFMailComposeViewController* picker = [[MFMailComposeViewController alloc] init];
picker.mailComposeDelegate = self;          
[picker setSubject:title];
[picker setMessageBody:[NSString stringWithFormat:[self emailBody], title, [link absoluteString]] isHTML:YES];
[self.viewController presentModalViewController:picker animated:YES];

当用户发送/取消时,将调用以下委托:

- (void) mailComposeController:(MFMailComposeViewController*)controller didFinishWithResult:(MFMailComposeResult)result error:(NSError*)error { 
    [self.viewController dismissModalViewControllerAnimated:YES];
}