将子视图控制器推送到UINavigationController后崩溃

Question

我正在尝试使用以下代码在堆栈上推送新的视图控制器：

RootViewController *childViewController = [[RootViewController alloc] init];
// initialize childViewController fields
[self.navigationController pushViewController:childViewController animated:YES];

当我这样做时，我得到以下崩溃。有人可以帮我找出原因吗？

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSMutableArray insertObject:atIndex:]: attempt to insert nil object at 1'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x00db0be9 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x00f055c2 objc_exception_throw + 47
    2   CoreFoundation                      0x00daa7f1 -[__NSArrayM insertObject:atIndex:] + 225
    3   CoreFoundation                      0x00da5c44 -[__NSArrayM addObject:] + 68
    4   UIKit                               0x003123fd -[UINavigationBar pushNavigationItem:] + 210
    5   UIKit                               0x00312947 -[UINavigationBar _pushNavigationItem:transition:] + 124
    6   UIKit                               0x003778f0 -[UINavigationController pushViewController:transition:forceImmediate:] + 1110
    7   UIKit                               0x003704a0 -[UINavigationController pushViewController:animated:] + 62
...

Answer 1

问题是该控制器的UINavigationItem为零。我有同样的确切崩溃，因为我正在返回一个尚未设置的自定义导航项。我通过返回super的navigationItem来解决它，如果它是nil：

- (UINavigationItem *) navigationItem
{
    if (navItem) {
        return navItem;
    }

    return [super navigationItem];
}