Current Variable is not valid

I am still battling a way to make a simple chart that shows aggregated amount of payments (transactions) monthly - reference to this post: https://community.flutterflow.io/ask-the-community/post/simple-chart-that-shows-aggregated-transactions-per-month-vq93degvPO1LgJA

Have created this custom function, that accepts list of documents returned from the query firestore, then does aggregation, and returns JSON.

List<dynamic>? getMonthlyPaymentTotals(List<PaymentsTestRecord> paymentsList) {
  /// MODIFY CODE ONLY BELOW THIS LINE

  // Create a map to store monthly total payments
  Map<String, double> monthlyTotals = {};

  // Iterate through each payment document
  for (PaymentsTestRecord payment in paymentsList) {
    // Extract the payment amount and created date
    double amount = payment.amount ?? 0.0;
    DateTime createdAt = payment.createdAt ?? DateTime.now();

    // Get the month and year from the created date
    String month = DateFormat('MM').format(createdAt);

    // Update the monthly total payments map
    if (monthlyTotals.containsKey(month)) {
      // Use the null assertion operator '!' to assert that the value is not null
      monthlyTotals[month] = monthlyTotals[month]! + amount;
    } else {
      monthlyTotals[month] = amount;
    }
  }

  // Convert the monthly totals map to a list of JSON objects
  List<dynamic> monthlyTotalsList = monthlyTotals.entries.map((entry) {
    return {
      'month': entry.key,
      'paymentsAmount': entry.value,
    };
  }).toList();

  return monthlyTotalsList;

  /// MODIFY CODE ONLY ABOVE THIS LINE
}

The result is expected to be like this

[
  {'month': '09', 'paymentsAmount': 300.0},
  {'month': '10', 'paymentsAmount': 200.0},
]

So what I've done:

1) on the chart widget added backend Query Collection

2) Selected in Chart Data Data Source - Firestore Documents

3) Selecting "Custom Function" as Data - getMonthlyPaymentTotals

4) Selecting in the function arguments - the paymentsList (as I am getting it from the backend query)

It shows "Current variable is not valid"

I believe I am on the right path, but got stuck in the instruments of FF to figure out how to get it nailed.