我正在尝试将导航栏添加到我的tabbar视图控制器。但是，它显示在firstViewController中，但不显示在其他两个视图中。有什么理由不行吗？
self.firstVC = [[FirstViewController alloc] initWithNibName:@"FirstViewController" bundle:nil fileName:@"firstPlist"];
self.firstVC.tabBarItem.image = [UIImage imageNamed:@"first"];
self.firstNavBarController = [[UINavigationController alloc] initWithRootViewController:self.firstVC];
self.secondVC = [[FirstViewController alloc] initWithNibName:@"SecondViewController" bundle:nil fileName:@"secondPlist"];
self.secondVC.tabBarItem.image = [UIImage imageNamed:@"second"];
self.secondNavBarController = [[UINavigationController alloc] initWithRootViewController:self.secondVC];
self.thirdVC = [[ThirdViewController alloc] initWithNibName:@"thirdView" bundle:nil fileName:@"thirdPlist"];
self.thirdVC.tabBarItem.image = [UIImage imageNamed:@"third"];
self.thirdNavBarController = [[UINavigationController alloc] initWithRootViewController:self.thirdVC];