OTPTextField: Null check operator used on a null value

I am using otp_text_field: ^1.1.3 to verify email by pin code in the Flutter app.
If I try to enter the very first value in the input box, the app crashes on exception.

App UI:

Code Editor:

Code:

import 'package:flutter/material.dart';
import 'package:flutter_countdown_timer/flutter_countdown_timer.dart';
import 'package:otp_text_field/otp_field.dart';
import 'package:otp_text_field/otp_text_field.dart';
import 'package:otp_text_field/style.dart';

...

class OTPPage extends StatefulWidget {
  SingInController con;
  OTPPage(this.con);
  @override
  _PageState createState() => _PageState();
}

class _PageState extends State<OTPPage> {
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Theme.of(context).backgroundColor,
        appBar: AppBar(
          elevation: 0,
          backgroundColor: Theme.of(context).backgroundColor,
        ),
        body: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            const SizedBox(
              height: 50,
            ),
            HeaderTxtWidget(
              'Enter the code from your email',
              fontSize: 16,
            ),
            const SizedBox(
              height: 20,
            ),
            Center(
              child: OTPTextField(
                length: 4,
                width: MediaQuery.of(context).size.width - 50,
                fieldWidth: 50,
                style: const TextStyle(fontSize: 17),
                textFieldAlignment: MainAxisAlignment.spaceAround,
                fieldStyle: FieldStyle.box,
                outlineBorderRadius: 10,
                onCompleted: (pin) {
                  widget.con.OTP = pin;
                },
              ),
            ),
            ...
          ],
        ));
  }
}

Added code for better instead of screenshot. Please let me know the solutions. Thank you!!!

To avoid the error, it is necessary to include the onChanged: (s) {} callback, even if you do not plan to use it. Without this callback, the onChange property will be null and the error will occur.

              OTPTextField(
                length: 4,
                width: MediaQuery.of(context).size.width - 50,
                fieldWidth: 50,
                style: const TextStyle(fontSize: 17),
                textFieldAlignment: MainAxisAlignment.spaceAround,
                fieldStyle: FieldStyle.box,
                outlineBorderRadius: 10,
                onChanged: (val) {},
                onCompleted: (pin) {
                  widget.con.OTP = pin;
                },
              )