Deleting a record in Types does not alter data stored in any field in Customers.
You may want to use a drop down list instead of a pop up menu for this. With a drop down list, you can put it behind a "name" field from the Types table that is set up in behavior to prevent access when in browse mode. Give this field a non transparent fill color and the drop down list field will remain hidden behind the "name" field until the user clicks on it or tabs into it. Since the name field is from Types, deleting the related types field will result in an empty field that still obscures the ID number stored in the drop down.