AJAX :: Enable / Disable Field Validation Using RequireFieldValidator And ValidatorCalloutExtender?

I am creating a fairly straight forward form for collecting data about vendors. I have a requirement that says the email address is required for a certain payment method and not other payment methods. The way I currently have this implemented is using a dropdownlist for the payment method and a TextBox for the email. I have been using ValidatorCalloutExtenders on all the other input fields with great success, but this is the first case where I need to be able to turn the validator and extender behavior on and off based on a selection of the dropdownlist.

I have tried many things and have searched these forums and google until my hands and eyes got tired, but have yet to find success, hence this post. I am aware of the dispose and $create methods, and that is currently what I am trying to do, but after the create occurs, while the behavior has been created, it doesn't seem like it has been created completely or something. To implement this, I am using a function tied to the onChange Event Handler of the drop down list. The function essentially checks the dropdownlist selection value and if it is NOT "ACH", then it finds the behavior attached to the requiredfieldvalidator attached to the email textbox and calls the dispose method on that behavior, if the selection value IS "ACH" then the method calls

$create(AjaxControlToolkit.ValidatorCalloutBehavior, { "closeImageUrl": "/VendorMaintenanceForm/WebResource.axd?d=IJFIM0l1l2KBLbEjcc5eS12jNidTRT60rU5Tz_Vg2CIJnYpw0GWOKDcVthhDzk9qkoREy7KswErAqM-kEQdJ5c18thtoit9YPZB3oq-EKdE1&t=633129167627987742", "highlightCssClass":
"HighLight", "id": 'emVal', "warningIconImageUrl": "/VendorMaintenanceForm/WebResource.axd?d=IJFIM0l1l2KBLbEjcc5eS12jNidTRT60rU5Tz_Vg2CIJnYpw0GWOKDcVthhDzk9qpoLKUlLQVCOCBk1mUhtC-YfwF57Tw0d676WGnnWiYI41&t=633129167627987742", "width": "300px" }, null, null,
$get("ctl00_MainContent_RequiredFieldValidator5"));

The reason I say this, is because once the page loads, the default value is to have the validator disabled, then if I select the payment method that enables the validator, it seems to work fine, but then if I cycle through the payment method choices again, I end up with a null pointer exception to the TextBox element when the validator is trying to apply the highlightCssClass on form validation. So, to summarize, it seems like when the page loads, things get built correctly, but then when I start doing the dispose and $create, things are recreated correctly so that when the submit button click causes validation, the validator throws a null pointer exception.

I know it can be hard to debug this sort of thing using words, but I was just wondering if I am going about this the right way or if there is an easier way to conditionally enable/disable a requiredfieldvalidator and validatorcalloutextender.