当我选择来自UIDocumentInteractionController“presentOpenInMenuFromRect”方法的应用程序时，外部应用程序没有打开

Question

当我从列表中选择Dropbox时，没有发生任何事情。这是代码：

- (void)openIn:(NSURL *)fileFullPath {
    docController = [UIDocumentInteractionController interactionControllerWithURL:fileFullPath];
    [docController setDelegate:self];

    BOOL isOpen = [docController presentPreviewAnimated:YES];

    if (isOpen == NO) {
        BOOL isOpenIn = [docController presentOpenInMenuFromRect:CGRectMake(300, 300, 100, 100) inView:self.view animated:NO];

        if(isOpenIn == NO) {
            return ;
        }
    }
}