Hi 

Try this code:

    const PaymentScreen = require('point_of_sale.PaymentScreen');

    const Registries = require('point_of_sale.Registries');

    const PosDeResPaymentScreen = PaymentScreen => class extends PaymentScreen {

        //@Override

        async validateOrder(isForceValidate) {const currentPartner = this.currentOrder.get_client();

        if (currentPartner == null) {

                    await this.showPopup('ErrorPopup', {

   title: _t('Unknown'),

   body: _t("Choose Customer First."),

                });

        }

                await super.validateOrder(...arguments);

    }, Registries.Component.extend(PaymentScreen, PosDeResPaymentScreen);

    return PaymentScreen;

});

Hope it helps