检测视图控制器是否退出

Question

我有一段代码用来检测ViewController，具体是在另一个Navigation Stack中的VC弹出时执行：

- (void) leftViewDidHide{
    if ([((AppDelegate *)[UIApplication sharedApplication].delegate).frontViewController.navigationController.viewControllers objectAtIndex:1]) {
    SGServerListViewController *sample = [[[((AppDelegate *)[UIApplication sharedApplication].delegate).frontViewController.navigationController.viewControllers objectAtIndex:1]childViewControllers] objectAtIndex:0];
    [sample.serverTableView setUserInteractionEnabled:YES];
    }
}

然而运行之后程序就会崩溃，异常出在if语句哪行。不知道怎么解决这个问题？我只需要检测是否这个视图控制器在执行代码，谢谢帮忙

补充：

异常是： out of bounds

Answer 1

- (void) leftViewDidHide{

if ([((AppDelegate *)[UIApplication sharedApplication].delegate).frontViewController.navigationController.viewControllers count] == 1) {
SGServerListViewController *sample = [[[((AppDelegate *)[UIApplication sharedApplication].delegate).frontViewController.navigationController.viewControllers objectAtIndex:1]childViewControllers] objectAtIndex:0];
[sample.serverTableView setUserInteractionEnabled:YES];
}

}