in the file \Templates\Designs\Swift\eCom7\CartV2\Step\Shared\Helpers\AddressDeliveryAnonymous_v2.cshtml, where your trying to get the ID of the checkoutpage
string userCheckoutPage = Pageview.CurrentParagraph.Item["CheckoutUserPageLink"] != null ? Pageview.CurrentParagraph.Item["CheckoutUserPageLink"].ToString().Replace("Default.aspx?Id=", "") : "";
The replace does not work.
the CheckoutUserPageLink is returned as Default.aspx?ID=xx, so it wille never be replaced.
Simple fix with null checks
string userCheckoutPage = Pageview.CurrentParagraph.Item["CheckoutUserPageLink"] != null ? Pageview.CurrentParagraph.Item["CheckoutUserPageLink"]?.ToString()?.ToLower().Replace("default.aspx?id=", "") ?? "" : "";
