MyOrderController.cs

public class MyOrderController : RenderController
{
    ...

    public override IActionResult Index()
    {
        ...

        if (orderId is not null)
        {
            // Since there's an order ID present we load it to
            // send the details to the view 
            // (what's the worst that could happen)
            var orderPage = (MyOrder) CurrentPage;
            orderPage.Order = _ordersService.GetOrder(member);
        }
        
        // If we can't find the order we just don't set it
        // and the template will handle the null and say
        // Sorry, we couldn't find your order :(
        return CurrentTemplate(CurrentPage);
    }
}