[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

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