NullReferenceException when trying to go to previous storyboard - xamarin.ios

Question

a bit of an iOS beginner here. I am tasked with creating a button that will navigate to the previous view controller. I have tried a lot of different ways, but the most recent is as follows:
partial void ButtonPressedPreviousOne(UIButton sender)
{
UIStoryboard HomeStoryboard = UIStoryboard.FromName("Home", null);
UserProfileViewController prevController = HomeStoryboard.InstantiateViewController("User_Profile_VC") as UserProfileViewController;
NavigationController.PopToViewController(prevController, true);
}
This throws a null reference exception. I only partially understand what is meant by this. Is there something wrong with the above code or am I missing something entirely different?

Answer 1

iOS Navigation controller has method NavigationController.PopViewController(true).
Try this one. But I strongly recommended check stack before do this like
public bool CanGoBack => NavigationController.ViewControllers.Length > 1;

Answer 2

Let see how navigation works:
Push:
To move from one page to another, an application will push a new page
onto the navigation stack, where it will become the active page, as
shown in the following diagram:
When we push a page, we usually use:
NavigationController.PushViewController();
Pop:
To return back to the previous page, the application will pop the
current page from the navigation stack, and the new topmost page
becomes the active page, as shown in the following diagram:
When we pop a page:
If you want to go back to previous page, use:
NavigationController.PopViewController(true);
If you want to go back to root page, use:
NavigationController.PopToRootViewController(true);
If you want to go back to specific page in Navigation Stack, use:
NavigationController.PopToViewController(viewController, true);
Refer : hierarchical Navigation
Back to your code:
If you use PopToViewController, the first paramater(prevController in your code) should be a existing ViewController in Navigation Stack instead of a new created. You got a null reference exception because the prevController you created is not in your Navigation Stack.
In your case, you can use:
var navController = this.NavigationController;
foreach (var controller in navController.ViewControllers)
{
if (controller is UserProfileViewController)
{
NavigationController.PopToViewController(controller,true);
break;
}
}
Note: if UserProfileViewController is the previous page you want to pop, use NavigationController.PopViewController(true); directly is simpler and faster.