Fix "undefined method new_record?" for user addresses

Fixed an issue where `new_record?` was called on a non-existent address
associated with an order object, leading to an "undefined method" error.
The problem was identified as the loss of a newly built address after a 
page refresh, caused by the address selection process retrieving the
address without assigning it to the order.

admin/app/controllers/solidus_admin/addresses_controller.rb

@@ -50,7 +50,8 @@ def update
 
     def find_address
       if params[:address_id].present? && @order.user
-        @order.user.addresses.find(params[:address_id])
+        address = @order.user.addresses.find_by(id: params[:address_id])
+        @order.send("#{address_type}_address=", address) if address
       else
         @order.send("#{address_type}_address")
       end