completer.complete(reply); - Problem while integrating Google Pay - flutter

Question

I am trying to integrate Google Pay payments into my app, and I see no errors in my code, but the Google Pay button is not showing up, and every time I hot reload my app, VSCode shows me
completer.complete(reply); this line of code. There seems to be no answer anywhere to this. Full code is attached below.
import 'package:flutter/material.dart';
import 'package:pay/pay.dart';
class HomePage extends StatelessWidget {
HomePage({Key? key}) : super(key: key);
final _paymentItems = <PaymentItem>[];
void onGooglePayResult(paymentResult) {
// Send the resulting Google Pay token to your server / PSP
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text("Ramzan Donation Form"),
),
body: Center(
child: Column(
children: [
const Text("Please check below to pay!"),
Row(children: [
GooglePayButton(
width: 200,
height: 50,
paymentConfigurationAsset: 'assets/gpay.json',
paymentItems: _paymentItems,
style: GooglePayButtonStyle.black,
type: GooglePayButtonType.pay,
margin: const EdgeInsets.only(top: 15.0),
onPaymentResult: (data) {
print(data);
},
loadingIndicator: const Center(
child: CircularProgressIndicator(),
),
),
])
],
),
),
);
}
}
I have got gpay.json under assets too. I have compared my code with working code online, I just don't get what's wrong. Any help is appreciated, thank you!