UIButton没有出现在iOS 5.1 iPad上

Question

UIBarButtonItem未出现在iOS 5.1 iPad 1上，但适用于iPad 4上的iOS 6。

_itemForImport = [[UIBarButtonItem alloc] initWithTitle:LS(@"Library", @"") style:UIBarButtonItemStyleBordered target:self action:@selector(itemForImportTouched)];


+ (void) presentInPopoverFromBarButtonItem:(UIBarButtonItem *)item withSourceType:(NSString*)sourceType  {

    UIPopoverController* popover = [IPadFilesViewController getPopoverController];
    if (popover)
        return;

    IPadImagePickerController* pickerController = [[IPadImagePickerController alloc] initWithSourceType:sourceType];

    popover = [[UIPopoverController alloc] initWithContentViewController:pickerController];
    popover.delegate = pickerController;
    [IPadFilesViewController setPopoverController:popover];

    [popover presentPopoverFromBarButtonItem:item permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
}