2 positional argument(s) expected, but 0 found bypass via another class

I created a class where I write a theme model:

import 'package:flutter/material.dart';
import 'theme_preference.dart';

class ThemeModel extends ChangeNotifier {
  bool _isDark;
  ThemePreferences _preferences;
  bool get isDark => _isDark;

  ThemeModel(this._preferences, this._isDark) {
    _isDark = false;
    _preferences = ThemePreferences();
    getPreferences();
  }

  set isDark(bool value) {
    _isDark = value;
    _preferences.setTheme(value);
    notifyListeners();
  }

  getPreferences() async {
    _isDark = await _preferences.getTheme();
    notifyListeners();
  }
}

to use the change I had to add create: (_) => ThemeModel(this._preferences, this._isDark), in the main class. But when I do it so, I get an error like this: The getter '_isDark' isn't defined for the type '_ToDoState'. I can´t initialise the variables in the main class so my question is, if there is a way that I can cast these thinks in the other class where I initialised it?

By the way I am following this tutorial:
https://www.flutterant.com/switching-themes-in-flutter-apps/

>Solution :

The main issue here is that ThemeModel is changed from the example in the article in an attempt to solve problems related to null-safety (the article is written before null-safety was introduced). Instead of the current ThemeModel it should be changed to:

class ThemeModel extends ChangeNotifier {
  bool _isDark = _false;
  ThemePreferences _preferences = ThemePreferences();
  bool get isDark => _isDark;

  ThemeModel() {
    getPreferences();
  }

  set isDark(bool value) {
    _isDark = value;
    _preferences.setTheme(value);
    notifyListeners();
  }

  void getPreferences() async {
    _isDark = await _preferences.getTheme();
    notifyListeners();
  }
}

Since the example is not made for ThemeModel to take any arguments.

Objects in Dart are created in two phases and all non-nullable non-late variable must have a value before the constructor body is executed. By changing the code like I have done, the variables _isDark and _preferences gets initial values before the constructor body is executing getPreferences();.