我目前正在用Xcode 6 (Beta 6)测试我的应用程序。UIActivityViewController在iPhone设备和模拟器上工作得很好，但在iPad模拟器和设备(iOS 8)上崩溃

Terminating app due to uncaught exception 'NSGenericException', 
reason: 'UIPopoverPresentationController 
(<_UIAlertControllerActionSheetRegularPresentationController: 0x7fc7a874bd90>) 
should have a non-nil sourceView or barButtonItem set before the presentation occurs.

我使用以下代码用于iPhone和iPad的iOS 7以及iOS 8

NSData *myData = [NSData dataWithContentsOfFile:_filename];
NSArray *activityItems = [NSArray arrayWithObjects:myData, nil];
UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:nil applicationActivities:nil];
activityViewController.excludedActivityTypes = @[UIActivityTypeCopyToPasteboard];
[self presentViewController:activityViewController animated:YES completion:nil];

我得到一个类似的崩溃在我的另一个应用程序以及。你能引导我吗?ios8中的UIActivityViewController有什么变化吗?我查过了，但在这上面什么也没找到