将navcontroller的根视图控制器设置为self时访问不正确

Question

执行此操作时，我收到错误的访问错误。第三行抛出错误。为什么呢？

UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Signup" bundle:nil];
AddFriendsTableViewController * svc =[sb instantiateViewControllerWithIdentifier:@"addFriendsController"];
UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:self];