How to load data from offline SQLite database to Flutter Calendar?

syncfusion_flutter_calendar: 18.4.39
sqflite: ^1.3.2+3
path_provider: ^1.6.27
intl: ^0.16.0
http: "0.11.3+17"
date_format: ^1.0.8
connectivity: ^0.3.0

static final table = 'flutter_calendar_events';
 
static final columnId = '_id';
static final columnName = 'name';
static final columnStart = 'start';
static final columnEnd = 'end';
 
// make this a singleton class
DatabaseHelper._privateConstructor();
static final DatabaseHelper instance = DatabaseHelper._privateConstructor();
 
// only have a single app-wide reference to the database
static Database _database;
Future<Database> get database async {
  if (_database != null) return _database;
  // lazily instantiate the db the first time it is accessed
  _database = await _initDatabase();
  return _database;
}
_initDatabase() async {
  Directory documentsDirectory = await getApplicationDocumentsDirectory();
  String path = join(documentsDirectory.path,”flutter_calendar_database.db”);
  return await openDatabase(path,
      version: _databaseVersion,
      onCreate: _onCreate);
}

// SQL code to create the database table
Future _onCreate(Database db, int version) async {
  await db.execute('''
        CREATE TABLE $table (
          $columnId INTEGER PRIMARY KEY,
          $columnName TEXT NOT NULL,
          $columnStart TEXT NOT NULL,
          $columnEnd TEXT NOT NULL
        )
        ''');
}

Future<int> insert(Map<String, dynamic> row) async {
  Database db = await instance.database;
  return await db.insert(table, row);
}
 
// All of the rows are returned as a list of maps, where each map is
// a key-value list of columns.
Future<List<Map<String, dynamic>>> queryAllRows() async {
  Database db = await instance.database;
  return await db.query(table);
}
 
// All of the methods (insert, query, update, delete) can also be done using
// raw SQL commands. This method uses a raw query to give the row count.
Future<int> queryRowCount() async {
  Database db = await instance.database;
  return Sqflite.firstIntValue(await db.rawQuery('SELECT COUNT(*) FROM $table'));
}
 
// We are assuming here that the id column in the map is set. The other
// column values will be used to update the row.
Future<int> update(Map<String, dynamic> row) async {
  Database db = await instance.database;
  int id = row[columnId];
  return await db.update(table, row, where: '$columnId = ?', whereArgs: [id]);
}
 
// Deletes the row specified by the id. The number of affected rows is
// returned. This should be 1 as long as the row exists.
Future<int> delete(int id) async {
  Database db = await instance.database;
  return await db.delete(table, where: '$columnId = ?', whereArgs: [id]);
}
 
Future<List> getAllRecords(String dbTable) async {
  var dbClient = await instance.database;
  var result = await dbClient.rawQuery("SELECT * FROM $dbTable");
 
  return result.toList();
}

DatabaseHelper dbHelper = DatabaseHelper.instance;
List<Color> _colorCollection;
List<String> _views;
 
@override
void initState() {
  _initializeEventColor();
  _views = <String>[
    'Insert',
    'Query',
    'Update',
    'Delete',
  ];
  super.initState();
}

appBar: AppBar(
    title: Text('SQL data'),
    leading: PopupMenuButton<String>(
      icon: Icon(Icons.calendar_today),
      itemBuilder: (BuildContext context) => _views.map((String choice) {
        return PopupMenuItem<String>(
          value: choice,
          child: Text(choice),
        );
      }).toList(),
      onSelected: (String value) {
        setState(() {
          if (value == 'Insert') {
            _insert();
          } else if (value == 'Query') {
            _query();
          } else if (value == 'Update') {
            _update();
          } else {
            _delete();
          }
        });
      },
    )),
body: FutureBuilder<List>(
    future: dbHelper.getAllRecords("flutter_calendar_events"),
    builder: (context, snapshot) {
      List<Appointment> collection = <Appointment>[];
  if (snapshot.data != null) {
    return ListView.builder(
      itemCount: 1,
      itemExtent: 550,
      itemBuilder: (context, int position) {
        var item = snapshot.data[position];
        Random random = new Random();
        for (int i = 0; i < snapshot.data.length; i++) {
          collection.add(
            Appointment(
              subject: item.row[1],
              startTime: _convertDateFromString(item.row[2]),
              endTime: _convertDateFromString(item.row[3]),
              color: _colorCollection[random.nextInt(9)],
            ),
          );
        }
        return SfCalendar(
          view: CalendarView.month,
          initialDisplayDate: DateTime(2021, 1, 4, 9, 0, 0),
          monthViewSettings: MonthViewSettings(showAgenda: true),
          dataSource: _getCalendarDataSource(collection),
        );
      },
    );
  } else {
    return CircularProgressIndicator();
  }
}));
void _initializeEventColor() {
  // ignore: deprecated_member_use
  _colorCollection = List<Color>();
  _colorCollection.add(const Color(0xFF0F8644));
  _colorCollection.add(const Color(0xFF8B1FA9));
  _colorCollection.add(const Color(0xFFD20100));
  _colorCollection.add(const Color(0xFFFC571D));
  _colorCollection.add(const Color(0xFF36B37B));
  _colorCollection.add(const Color(0xFF01A1EF));
  _colorCollection.add(const Color(0xFF3D4FB5));
  _colorCollection.add(const Color(0xFFE47C73));
  _colorCollection.add(const Color(0xFF636363));
  _colorCollection.add(const Color(0xFF0A8043));
}