Xamarin Forms - CustomRenderer Not Working After Navigating Away

I have a custom renderer in my iOS Native code for my main page. It works perfectly fine when the app start up, and renders Navbar items using the iOS System icons which is what I want. However, if I navigate away from the main page, when I navigate back the RightBarButtonItems array only contains two uninstantiated objects, I put in a check (RightNavItems.Title == null) to continue when this was the case to see what would happen, and indeed the items are not rendered, if I navigate away and back again the app crashes since the RightBarButtonItems array is now empty.

Why is it that the toolbar items are uninitialised when navigating back to the main page? What is the proper way to deal with navigation in a custom renderer like this?

Here is the code for the custom renderer:

public class ItemsPageRenderer : PageRenderer
{
    public new ItemsPage Element
    {
        get { return (ItemsPage)base.Element; }
    }

public override void ViewWillAppear(bool animated)
{
    base.ViewWillAppear(animated);

    var rightNavList = new List<UIBarButtonItem>();

    var navigationItem = this.NavigationController.TopViewController.NavigationItem;

    for (var i = 0; i < Element.ToolbarItems.Count; i++)
    {
        var reorder = (Element.ToolbarItems.Count - 1);
        var ItemPriority = Element.ToolbarItems[reorder - i].Priority;

        UIBarButtonItem RightNavItems = navigationItem.RightBarButtonItems[i];
        if (RightNavItems.Title == null)
            continue;

        if (RightNavItems.Title.ToLower() == "add")
        {
            rightNavList.Add(new UIBarButtonItem(UIBarButtonSystemItem.Add)
            {
                Action = RightNavItems.Action,
                Target = RightNavItems.Target
            });
        }
        else if (RightNavItems.Title.ToLower() == "edit")
        {
            rightNavList.Add(new UIBarButtonItem(UIBarButtonSystemItem.Edit)
            {
                Action = RightNavItems.Action,
                Target = RightNavItems.Target
            });
        }
        else
        {
            rightNavList.Add(RightNavItems);
        }
    }

    navigationItem.SetRightBarButtonItems(rightNavList.ToArray(), false);
}
}

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