LateInitializationError: Field '_databaseHelper@26286205' has not been initialized – Flutter

I am having Late initialization error. I am facing this issue from long time and I tried all possible ways to sort it out but, I failed can anyone help?

==============================
database_helper.dart
==============================
import 'Note.dart';
import 'package:sqflite/sqflite.dart';
import 'dart:async';
import 'dart:io';
import 'package:path_provider/path_provider.dart';

class DatabaseHelper {
  static late DatabaseHelper _databaseHelper; //Singleton
  static late Database _database; // Singleton

  String noteTable = 'note_table';
  String colId = 'id';
  String colTitle = 'title';
  String colDescription = 'description';
  String colPriority = 'priority';
  String colDate = 'date';

  DatabaseHelper._createInstance();

  factory DatabaseHelper() {
    if (DatabaseHelper == null) {
      _databaseHelper = DatabaseHelper._createInstance();
    }
    return _databaseHelper;
  }
  // custom getter
  Future<Database> get database async {
    if (_database == null) {
      _database = await initializeDatabase();
    }
    return _database;
  }

  Future<Database> initializeDatabase() async {
    Directory directory = await getApplicationDocumentsDirectory();
    String path = directory.path + 'note.db';

    var notesDatabase = await openDatabase(
      path,
      version: 1,
      onCreate: _createDB,
    );
    return notesDatabase;
  }

  void _createDB(Database db, int newVersion) async {
    await db.execute(
        'CREATE TABLE $noteTable($colId INTEGER PRIMARY KEY AUTOINCREMENT,$colTitle TEXT,$colDescription TEXT, $colPriority INTEGER,$colDate TEXT)');
  }

  Future<List<Map<String, dynamic>>> getNoteMapList() async {
    Database db = await this.database;
    // regular MySQL query optional
    // var result =
    //     await db.rawQuery('SELECT * from $noteTable order by $colPriority ASC');
    var result = await db.query(noteTable, orderBy: '$colPriority ASC');
    return result;
  }

  Future<int> insertNote(Note note) async {
    Database db = await this.database;
    var result = await db.insert(noteTable, note.toMap());
    return result;
  }

  Future<int> updateNote(Note note) async {
    Database db = await this.database;
    var result = await db.update(noteTable, note.toMap(),
        where: '$colId=?', whereArgs: [note.id]);
    return result;
  }

  Future<int> deleteNote(int id) async {
    Database db = await this.database;
    var result = await db.rawDelete('DELETE FROM $noteTable where $colId=$id');
    return result;
  }

  Future<int?> getCount() async {
    Database db = await this.database;
    List<Map<String, dynamic>> x =
        await db.rawQuery('SELECT COUNT (*) from $noteTable');
    int? result = Sqflite.firstIntValue(x);
    return result;
  }

  Future<List<Note>> getNoteList() async {
    var noteMapList = await getNoteMapList();
    int count = noteMapList.length;
    List<Note> noteList = <Note>[];
    for (var i = 0; i < count; i++) {
      noteList.add(Note.fromMapObject(noteMapList[i]));
    }
    return noteList;
  }
}

Following is note_detail.dart which is using database helper capabilities. I have created some methods and getters which is using database. it is also giving an error while I haven’t use "return" in WillPopScope().

======================
note_detail.dart
======================

import 'package:flutter/material.dart';
import '../Note.dart';
import '../database_helper.dart';
import 'package:intl/intl.dart';

class NoteDetail extends StatefulWidget {
  final String appBarTitle;
  final Note note;
  NoteDetail(this.note, this.appBarTitle);
  @override
  State<StatefulWidget> createState() {
    return NoteDetailState(this.note, this.appBarTitle);
  }
}

class NoteDetailState extends State<NoteDetail> {
  static var _priorities = ['High', 'Low'];
  DatabaseHelper helper = DatabaseHelper();
  String appBarTitle;
  Note note;

  NoteDetailState(this.note, this.appBarTitle);

  TextEditingController titleController = TextEditingController();
  TextEditingController descriptionController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    TextStyle? textStyle = Theme.of(context).textTheme.titleMedium;
    titleController.text = note.title;
    descriptionController.text = note.description!;

    return WillPopScope(
      onWillPop: () async {
        bool? result = await movetoLastScreen();
        result ??= false;
        return result;
      },
      child: Scaffold(
        backgroundColor: Colors.cyanAccent,
        appBar: AppBar(
          title: Text(appBarTitle),
          backgroundColor: Colors.pink,
          leading: IconButton(
            icon: Icon(Icons.arrow_back),
            onPressed: () {
              movetoLastScreen();
            },
          ),
        ),
        body: Padding(
          padding: EdgeInsets.all(10.0),
          child: Card(
            shape: RoundedRectangleBorder(
              borderRadius: BorderRadius.circular(15.0),
            ),
            child: ListView(
              children: <Widget>[
                Padding(
                  padding: EdgeInsets.only(top: 15.0, bottom: 5.0),
                  //dropdown menu
                  child: new ListTile(
                    leading: const Icon(Icons.low_priority),
                    title: DropdownButton(
                        items: _priorities.map((String dropDownStringItem) {
                          return DropdownMenuItem<String>(
                            value: dropDownStringItem,
                            child: Text(dropDownStringItem,
                                style: TextStyle(
                                    fontSize: 20.0,
                                    fontWeight: FontWeight.bold,
                                    color: Colors.red)),
                          );
                        }).toList(),
                        value: getPriorityAsString(note.priority),
                        onChanged: (valueSelectedByUser) {
                          setState(() {
                            updatePriorityAsInt(valueSelectedByUser!);
                          });
                        }),
                  ),
                ),
                // Second Element
                Padding(
                  padding: EdgeInsets.only(top: 15.0, bottom: 15.0, left: 15.0),
                  child: TextField(
                    controller: titleController,
                    style: textStyle,
                    onChanged: (value) {
                      updateTitle();
                    },
                    decoration: InputDecoration(
                      labelText: 'Title',
                      labelStyle: textStyle,
                      icon: Icon(Icons.title),
                    ),
                  ),
                ),

                // Third Element
                Padding(
                  padding: EdgeInsets.only(top: 15.0, bottom: 15.0, left: 15.0),
                  child: TextField(
                    controller: descriptionController,
                    style: textStyle,
                    onChanged: (value) {
                      updateDescription();
                    },
                    decoration: InputDecoration(
                      labelText: 'Details',
                      icon: Icon(Icons.details),
                    ),
                  ),
                ),

                // Fourth Element
                Padding(
                  padding: EdgeInsets.all(15.0),
                  child: Row(
                    children: <Widget>[
                      Expanded(
                        child: ElevatedButton(
                          child: Text(
                            'Save',
                            textScaleFactor: 1.5,
                            style: TextStyle(
                              color: Colors.white,
                            ),
                          ),
                          style: ElevatedButton.styleFrom(
                            backgroundColor: Colors.red,
                            padding: const EdgeInsets.all(8.0),
                          ),
                          onPressed: () {
                            setState(() {
                              debugPrint("Save button clicked");
                              _save();
                            });
                          },
                        ),
                      ),
                      Container(
                        width: 5.0,
                      ),
                      Expanded(
                        child: ElevatedButton(
                          child: Text(
                            'Delete',
                            textScaleFactor: 1.5,
                            style: TextStyle(
                              color: Colors.white,
                            ),
                          ),
                          style: ElevatedButton.styleFrom(
                            backgroundColor: Colors.red,
                            padding: const EdgeInsets.all(8.0),
                          ),
                          onPressed: () {
                            setState(() {
                              _delete();
                            });
                          },
                        ),
                      ),
                    ],
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }

  void updateTitle() {
    note.title = titleController.text;
  }

  void updateDescription() {
    note.description = descriptionController.text;
  }

  void _save() async {
    movetoLastScreen();
  }

  void _delete() async {
    movetoLastScreen();

    if (note.id == null) {
      _showAlertDialog('Status', 'First add a note');
      return;
    }

    int? result = await helper.deleteNote(note.id);
    if (result != 0) {
      _showAlertDialog('Status', 'Note deleted Successfully');
    } else {
      _showAlertDialog('Status', 'Error');
    }
  }

  // convert to int to save into database
  void updatePriorityAsInt(String value) {
    switch (value) {
      case 'High':
        note.priority = 1;
        break;
      case 'Low':
        note.priority = 2;
        break;
    }
  }

  // convert int to String to show user
  String getPriorityAsString(int value) {
    late String priority;
    switch (value) {
      case 1:
        priority = _priorities[0];
        break;
      case 2:
        priority = _priorities[1];
        break;
    }
    return priority;
  }

  movetoLastScreen() async {
    Navigator.pop(context, true);

    note.date = DateFormat.yMMMd().format(DateTime.now());
    int? result;
    if (note.id != null) {
      result = await helper.updateNote(note);
    } else {
      result = await helper.insertNote(note);
    }
    if (result != 0) {
      _showAlertDialog('Status', 'Note Save Successfully');
    } else {
      _showAlertDialog('Status', 'Problem saving Note');
    }
  }

  void _showAlertDialog(String title, String message) {
    AlertDialog alertDialog = AlertDialog(
      title: Text(title),
      content: Text(message),
    );
    showDialog(context: context, builder: (_) => alertDialog);
  }
}

I have tried every possible whether to check Null or async issue but nothing worked.