Can't receive object instance from ChangeNotifierProvider - flutter

Question

I have this code:
import 'package:flutter/foundation.dart';
import 'package:mqtt_client/mqtt_server_client.dart';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:finalprojectapp/Providers/Message_provider.dart';
import 'package:finalprojectapp/Providers/Client_provider.dart';
class MQTTManager with ChangeNotifier{
//Properties
MqttServerClient client;
MQTTMessageProvider messageProvider = MQTTMessageProvider();
String _identifier;
String _topic;
String _host;
//Getters
//Setters
Future initialize({String host, String identifier}) async {
MqttServerClient _client = MqttServerClient(host, identifier);
this._identifier = _client.clientIdentifier;
this._host = host;
_client.port = 1883;
_client.keepAlivePeriod = 20;
_client.onDisconnected = onDisconnected;
_client.onConnected = onConnected;
_client.onSubscribed = onSubscribed;
_client.logging(on: false);
final conMess = MqttConnectMessage()
.withClientIdentifier(identifier)
.keepAliveFor(20)
.withWillTopic('willtopic')
.withWillMessage('willmessage')
.startClean()
.withWillQos(MqttQos.atLeastOnce);
_client.connectionMessage = conMess;
try {
MqttClientConnectionStatus result = await _client.connect('BBFF-qkHkFkvJ6oFUw9m6Pa9bzQTCbVCddH','');
this.client = _client;
notifyListeners();
return result.state;
} on Exception catch (e) {
print('Something went wrong $e');
disconnect();
return null;
}
}
void subscription({String topic}) {
this._topic = topic;
print('EXAMPLE::Subscribing to the $_topic topic');
this.client.subscribe(this._topic, MqttQos.atMostOnce);
}
void unsubscribe({String topic}) {
print('unsubscribing from $topic');
this.client.unsubscribe(topic);
print('Unsubscribbed!');
}
void publish({String topic, String message}) async {
final builder = MqttClientPayloadBuilder();
builder.addString(message);
this.client.publishMessage(topic, MqttQos.atMostOnce, builder.payload);
}
void disconnect() async {
await MqttUtilities.asyncSleep(2);
print('EXAMPLE::Disconnecting');
this.client.disconnect();
}
/// The subscribed callback
void onSubscribed(String topic) {
this._topic = topic;
print('EXAMPLE::Subscription confirmed for topic $this._topic');
this.client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
final MqttPublishMessage _recMess = c[0].payload;
final String _message =
MqttPublishPayload.bytesToStringAsString(_recMess.payload.message);
messageProvider.setMessage(_message);
});
}
/// The unsolicited disconnect callback
void onDisconnected() {
print('EXAMPLE::OnDisconnected client callback - Client disconnection');
if (this.client.connectionStatus.returnCode == MqttConnectReturnCode.solicited) {
print('EXAMPLE::OnDisconnected callback is solicited, this is correct');
}
}
/// The successful connect callback
void onConnected() {
print(
'EXAMPLE::OnConnected client callback - Client connection was sucessful');
}
/// Pong callback
void pong() {
print('EXAMPLE::Ping response client callback invoked');
}
}
Which is in charge of notifying when ive obtained a client from Mqtt broker, and is then passed to the client property of this class. When this happens, notifyListeners is supposed to notify this Provider:
class AppWrapper extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider<MQTTMessageProvider>(
create: (_) => MQTTMessageProvider(),
),
ChangeNotifierProvider<MQTTManager>(
create: (_) => MQTTManager(),
),
],
child: MQTTInitialize()
);
}
}
and then retrieve it in this class ( which is child of MQTTInitialize() ) :
final MQTTManager managerProvider = Provider.of<MQTTManager>(context);
return Scaffold(
appBar: AppBar(
backgroundColor: Colors.blueGrey,
title: Text('Subscribe'),
actions: <Widget>[
FlatButton.icon(
onPressed: (){
managerProvider.disconnect();
},
icon: Icon(Icons.arrow_back),
label: Text('return'),
),
],
),
... (it continues)
Problem is that, when I press the button, it throws the following error:
[ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'disconnect' was called on null.
E/flutter (30793): Receiver: null
E/flutter (30793): Tried calling: disconnect()
E/flutter (30793): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
Which I suppose is because managerProvider in final MQTTManager managerProvider = Provider.of<MQTTManager>(context); is null, so I can't call managerProvider.disconnect();
How can I get my ChangeNotifierProvider to provide a correct instance of manager?
Link to project: https://github.com/TacoMariachi/Mqtt_flutter_app.git