Flutter – Firebase call exception

I’m trying to make simple login logic using Firebase but can’t get rid of this error:

Exception has occurred.
DartError: Bad state: Cannot fire new event. Controller is already firing an event

Here is my code:

import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
import 'package:identity_retro/models/user_model.dart';
import 'package:uuid/uuid.dart';

class DatabaseOperations {
  static final FirebaseDatabase database = FirebaseDatabase.instance;
  DatabaseReference ref = FirebaseDatabase.instance.ref();

  Future<UserModel?> getUserByEmail(String username) async {
    var response = await ref
        .child('users')
        .orderByChild('username')
        .equalTo(username)
        .once();

    if (!response.snapshot.exists) {
      return null;
    }

    var userMap = Map<String, dynamic>.from(response.snapshot.value as Map);
    var user =
        UserModel.fromJson(userMap);

    return user;
  }

  Future<UserModel?> getUserById(String id) async {
    var response =
        await ref.child('users').orderByChild('id').equalTo(id).once();

    if (!response.snapshot.exists) {
      return null;
    }

    var user =
        UserModel.fromJson(response.snapshot.value as Map<String, dynamic>);

    return user;
  }

  Future<bool> addUser(UserModel user, BuildContext context) async {
    if (await getUserByEmail(user.username!) != null)
    {
      if (context.mounted) {
        await showDialog(
            context: context,
            builder: (BuildContext context) {
              return AlertDialog(
                title: const Text('Error'),
                content: const Text('User already exists'),
                actions: <Widget>[
                  TextButton(
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                    child: const Text('OK'),
                  ),
                ],
              );
            });
      }

      return false;
    }

    var id = Uuid().v4();

    await ref.child('users/$id').set({'username': user.username, 'password': user.password});

    return await getUserById(id) != null;
  }
}

Exception happens in positive scenario when user doesn’t exists and it trying to make new one in this line:

await ref.child('users/$id').set({'username': user.username, 'password': user.password});

Stacktrace leads to database_reference_web.dart file line 51. It’s a set method

But it worth to mention than despite on the error user is created after all