LateInitializationError: Field 'dialogflow' has not been initialized

My code works fine last week, then when the credentials expire, i recreate another credentials and replace the credentials, now the code doesnt work anymore and show the error
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: LateInitializationError: Field ‘dialogflow’ has not been initialized.

import 'package:dialogflow_grpc/dialogflow_grpc.dart';
import 'package:dialogflow_grpc/generated/google/cloud/dialogflow/v2beta1/session.pb.dart';
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class Chat extends StatefulWidget {
  Chat({Key? key}) : super(key: key);
  @override
  _ChatState createState() => _ChatState();
}
class _ChatState extends State<Chat> {
  final List<ChatMessage> _messages = <ChatMessage>[];
  final TextEditingController _textController = TextEditingController();

  late DialogflowGrpcV2Beta1 dialogflow;

  @override
  void initState() {
    super.initState();
    initPlugin();
  }

  Future<void> initPlugin()async{
    final serviceAccount = ServiceAccount.fromString(
        '${(await rootBundle.loadString('assets/credentials2.json'))}');
    // Create a DialogflowGrpc Instance
    dialogflow = DialogflowGrpcV2Beta1.viaServiceAccount(serviceAccount);
  }
  void handleSubmitted(text) async {
    print(text);
    _textController.clear();
    ChatMessage message = ChatMessage(
      text: text,
      name: "You",
      type: true,
    );

    setState(() {
      _messages.insert(0, message);
    });

    DetectIntentResponse data = await dialogflow.detectIntent(text, 'en-US');
    String fulfillmentText = data.queryResult.fulfillmentText;
    if(fulfillmentText.isNotEmpty) {
      ChatMessage botMessage = ChatMessage(
        text: fulfillmentText,
        name: "Bot",
        type: false,
      );

      setState(() {
        _messages.insert(0, botMessage);
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Chatbot Page'),
      ),
      body: Column(children: <Widget>[
        Flexible(
            child: ListView.builder(
              padding: EdgeInsets.all(8.0),
              reverse: true,
              itemBuilder: (_, int index) => _messages[index],
              itemCount: _messages.length,
            )),
        Divider(height: 1.0),
        Container(
            decoration: BoxDecoration(color: Theme.of(context).cardColor),
            child: IconTheme(
              data: IconThemeData(color: Theme.of(context).accentColor),
              child: Container(
                margin: const EdgeInsets.symmetric(horizontal: 8.0),
                child: Row(
                  children: <Widget>[
                    Flexible(
                      child: TextField(
                        controller: _textController,
                        onSubmitted: handleSubmitted,
                        decoration: InputDecoration.collapsed(hintText: "Send a message to begin"),
                      ),
                    ),
                    Container(
                      margin: EdgeInsets.symmetric(horizontal: 4.0),
                      child: IconButton(
                        icon: Icon(Icons.send),
                        onPressed: () => handleSubmitted(_textController.text),
                      ),
                    ),
                  ],
                ),
              ),
            )
        ),
      ]),
    );
  }
}

class ChatMessage extends StatelessWidget {
  ChatMessage({required this.text, required this.name, required this.type});

  final String text;
  final String name;
  final bool type;

  List<Widget> otherMessage(context) {
    return <Widget>[
      new Container(
        margin: const EdgeInsets.only(right: 16.0),
        child: CircleAvatar(child: new Text('B')),
      ),
      new Expanded(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Text(this.name,
                style: TextStyle(fontWeight: FontWeight.bold)),
            Container(
              margin: const EdgeInsets.only(top: 5.0),
              child: Text(text),
            ),
          ],
        ),
      ),
    ];
  }

  List<Widget> myMessage(context) {
    return <Widget>[
      Expanded(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.end,
          children: <Widget>[
            Text(this.name, style: Theme.of(context).textTheme.subtitle1),
            Container(
              margin: const EdgeInsets.only(top: 5.0),
              child: Text(text),
            ),
          ],
        ),
      ),
      Container(
        margin: const EdgeInsets.only(left: 16.0),
        child: CircleAvatar(
            child: Text(
              this.name[0],
              style: TextStyle(fontWeight: FontWeight.bold),
            )),
      ),
    ];
  }
  @override
  Widget build(BuildContext context) {
    return Container(
      margin: const EdgeInsets.symmetric(vertical: 10.0),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: this.type ? myMessage(context) : otherMessage(context),
      ),
    );
  }
}

>Solution :

initPlugin is a future method, It will take some frame to initialize the dialogflow. You can make it nullable instead of using late,

DialogflowGrpcV2Beta1? dialogflow;

Now the place where you will use, you can do a null check.

Also a better way of doing this using FutureBuilder.

 Future<DialogflowGrpcV2Beta?> initPlugin()async{
    final serviceAccount = ServiceAccount.fromString(
        '${(await rootBundle.loadString('assets/credentials2.json'))}');
    // Create a DialogflowGrpc Instance
    return DialogflowGrpcV2Beta1.viaServiceAccount(serviceAccount);
  }

  late  final Future<DialogflowGrpcV2Beta> dialogflowFuture  = initPlugin();
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FutureBuilder(
        future:dialogflowFuture,
        builder: (context, snapshot) { 
         if(snapshot.hasError) return Text("Got Error");
         if (snapshot.hasData){
         final dialogflow = snapshot.data;
         return Column(children: <Widget>[...
         }
        return CircularProgressIndicator(); //last

More about using FutureBuilder