Flutter Serverpod returns 400 error on sample project

I'm newly learning Serverpod version 0.9.21 as my backend, first I changed the Client address from http://localhost:8080/ to http://10.0.2.2:8080/ so the SocketException error had gone on the emulator and now the sample app (say hello) works correctly. The Problem is my code faces Error 400 with no errorMessage, Also I've added the tables and checked the database, and everything looks fine.

After creating the project, I added Province Protocol inside the test_server package:

class: Province
table: province
fields:
  name: String
  isEnabled: bool

Then added the province_endpoint:

import 'package:serverpod/serverpod.dart';
import '../generated/protocol.dart';

class ProvinceEndpoint extends Endpoint {
  Future<bool> addProvince(Session session, Province province) async {
    await Province.insert(session, province);
    return true;
  }
}

Then run serverpod generate and start the docker and the server as tutorials are mentioning.

And finally the test_flutter main.dart:

import 'package:back_client/back_client.dart';
import 'package:flutter/material.dart';
import 'package:serverpod_flutter/serverpod_flutter.dart';

var client = Client('http://10.0.2.2:8080/')
  ..connectivityMonitor = FlutterConnectivityMonitor();

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final _nameController = TextEditingController(text: "name1");
  String result = "here";

  addProvince() async {
    final province = Province(name: _nameController.text, isEnabled: true);
    client.province.addProvince(province).then((value) {
      if (value) {
        setState(() {
          result = "Done adding ${_nameController.text} province";
        });
      } else {
        result = "Nashod!";
      }
    }).catchError((onError) {
      final error = (onError is ServerpodClientException)
          ? "${onError.message} ${onError.statusCode}"
          : "Error ";
      setState(() {
        result = error;
      });
    }).whenComplete(() {
      print("done");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: [
          TextField(
            controller: _nameController,
          ),
          Text(
            result,
            style: Theme.of(context).textTheme.headline3,
          )
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: addProvinces,
        child: const Icon(Icons.add),
      ),
    );
  }
}

I am a noob in Backend stuff, I didn't restart the docker and server after developing the endpoint! As flutter has hot-reload it made me a little bit lazy! After development, in the terminal that serverpod is running, press Ctrl + C to close its processes, then run docker-compose up --build --detach && dart bin/main.dart