Hello, I am getting the above error when I click on IsCustomButton button in POS Payment screen. 

Here is the full list of the error:
TypeError: utils.getComponent(...).IsCustomButton is not a function
at extra.handlers.click__1043__ (eval at _compile (http://localhost:8011/web/assets/1158- 26a83f0/ web.assets_common.min.js:1014:178), :350:174)
at invokeHandler (http://localhost:8011/web/assets/1158-26a83f0/ web.assets_common.min.js:838:144 )
at handleEvent (http://localhost:8011/web/assets/1158-26a83f0/ web.assets_common.min.js:842:92)
at HTMLDivElement.handler (http://localhost:8011/web/assets/1158 -26a83f0/ web.assets_common.min.js:844:58)

Here is the JS code:

odoo.define( 'pos_custom_buttons.PaymentScreenButton' , function (require) {
'use strict' ;
const { Gui } = require( 'point_of_sale.Gui' );
const PosComponent = require( 'point_of_sale.PosComponent' );
const { posbus } = require( 'point_of_sale.utils' );
const ProductScreen = require( 'point_of_sale.ProductScreen' );
const { useListener } = require( 'web.custom_hooks' );
const Registries = require( 'point_of_sale.Registries' );
const PaymentScreen = require( 'point_of_sale.PaymentScreen');
const CustomButtonPaymentScreen = (PaymentScreen) =>
class extends PaymentScreen {
constructor() {
super(...arguments);
}
IsCustomButton() {
let partnerId = this .rpc({
model: 'model.model' ,
method: 'do_payment' ,
args: [event.detail.processedChanges],
});

var phone = prompt( "Enter the Customer phone number" );
var send_phone = "1" + String(parseInt(phone))
alert( "Request sent to: " + send_phone)

console.log(phone)
// Gui.showPopup("ErrorPopup", {
// title: this.env._t('Send request'),
// body: this. env._t('Send request'),
// });
}
};
Registries.Component.extend(PaymentScreen, CustomButtonPaymentScreen);
return CustomButtonPaymentScreen;
});