flutter sqflite database resets when app is closed and reopened

I'm working on my database to persist notes when the app is closed (process killed). It worked before the null-safety implementation, however, now it deletes all the data when I close the app and reopen it.

Here is the code of my Database class:

import 'dart:io';
import 'package:notaria/database/note.dart';
import 'package:path/path.dart';
import 'package:path_provider/path_provider.dart';
import 'package:sqflite/sqflite.dart';

class NotesDatabase {
  static final _dbName = 'notes.db';
  static final _dbVersion = 1;

  NotesDatabase._privateConstructor();
  static final NotesDatabase instance = NotesDatabase._privateConstructor();

  static Database? _database;

  Future<Database> get database async {
    if (_database != null) {
      return _database!;
    }
    return _database = await _initDatabase();
  }

  Future<Database> _initDatabase() async {
    Directory directory = await getApplicationDocumentsDirectory();
    String path = join(directory.path, _dbName);
    print(path);
    return await openDatabase(path, version: _dbVersion, onCreate: _onCreate);
  }

  Future<void> _onCreate(Database db, int version) async {
    await db.execute('''
          CREATE TABLE notes (
            id TEXT,
            title TEXT,
            notetext TEXT,
            videourl TEXT,
            thumbnail TEXT
          )
          ''');
  }

  Future<List<Note>> getNotes() async {
    Database db = await instance.database;

    var notes = await db.query('notes');

    List<Note> noteList = notes.isNotEmpty
        ? notes.map((note) => Note.fromMap(note)).toList()
        : [];
    return noteList;
  }

  Future<int> insert(Note note) async {
    Database db = await instance.database;
    return await db.insert('notes', note.toMap());
  }

  Future<int> delete(int id) async {
    Database db = await instance.database;
    return await db.delete('notes', where: 'id = ?', whereArgs: [id]);
  }

  Future<int> update(Note note) async {
    Database db = await instance.database;
    return await db
        .update('notes', note.toMap(), where: 'id = ?', whereArgs: [note.id]);
  }
}

Is it possible that the database does not get opened properly? Or I overwrite the already existing database with empty text?

I'm thankful for any help.