Flutter and query postgress - postgresql

Question

I'm new with flutter, so, I trying to connect flutter project with postgress database across docker DBeaver and it's work, but only if the main is this:
void postgresConnect() async {
final conn = PostgreSQLConnection(
'localhost',
5432,
'postgres',
username: 'postgres',
password: '***',
);
await conn.open();
print('Connected to Postgres database...');
var results = await conn.query('SELECT name from historical');
print(results);
await conn.close();
}
void main(List<String> arguments) async {
postgresConnect();
}
But if the main is this:
void main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
runApp(MyApp());
}
And if inside one of my classes I enter the same code no longer works and gives me the following error messages:
E/flutter (19670): [ERROR:flutter/lib/ui/ui_dart_state.cc(198)]
Unhandled Exception: SocketException: Connection refused (OS Error:
Connection refused, errno = 111), address = localhost, port = 58886
E/flutter (19670): #0 _NativeSocket.startConnect
(dart:io-patch/socket_patch.dart:682:35) E/flutter (19670): #1
_NativeSocket.connect (dart:io-patch/socket_patch.dart:948:12) E/flutter (19670): #2 _RawSocket.connect
(dart:io-patch/socket_patch.dart:1805:26) E/flutter (19670): #3
RawSocket.connect (dart:io-patch/socket_patch.dart:21:23) E/flutter
(19670): #4 Socket._connect (dart:io-patch/socket_patch.dart:2028:22)
E/flutter (19670): #5 Socket.connect (dart:io/socket.dart:743:21)
E/flutter (19670): #6 PostgreSQLConnection.open
(package:postgres/src/connection.dart:160:32) E/flutter (19670): #7
postgresConnect (package:poi/pages/request.dart:18:14) E/flutter
(19670): #8 _RequestPageState._read
(package:poi/pages/request.dart:211:5) E/flutter (19670): #9
_RequestPageState.SaveLatLong (package:poi/pages/request.dart:185:5) E/flutter (19670): #10 _RequestPageState.build.
(package:poi/pages/request.dart:137:37) E/flutter (19670):