Insert dark theme in settings_ui dependencies

Question

I implemented settings_ui in dependencies on Flutter.

I'm trying to use a tile to set the theme but I get the error:

NoSuchMethodError (NoSuchMethodError: The method 'getBool' was called on null.
Receiver: null
Tried calling: getBool("theme"))

I don't know why I get this error since I initialized the bool value... Tell me if you need something else in comments.

settings.dart

void main() => runApp(Settings());

class Settings extends KFDrawerContent {
  @override
  _ImpostazioniAppState createState() => _ImpostazioniAppState();
}

class _SettingsState extends State<SettingsApp> {
  SharedPreferences _prefs;
  final String _keyTheme = "theme";
  bool darktheme = false;

  @override
  Widget build(BuildContext context) {
    bool temaDark = _prefs.getBool(_keyTheme) ?? true;

    return SettingsList(
      sections: [
        SettingsSection(
          title: Text('Common'),
          tiles: <SettingsTile>[
            SettingsTile.navigation(
              leading: Icon(Icons.language),
              title: Text('Language'),
              value: Text('English'),
            ),
            SettingsTile.switchTile(
              onToggle: (value) {
                _prefs.setBool(_keyTheme, value);
              },
              initialValue: darktheme,
              leading: Icon(Icons.format_paint),
              title: Text('Enable custom theme'),
            ),
          ],
        ),
      ],
    );
  }
}

theme_provider.dart

class ThemeNotifier extends ChangeNotifier {
  final String key = "theme";
  SharedPreferences _prefs;
  bool _darkTheme;

  bool get darkTheme => _darkTheme;

  ThemeNotifier() {
    _darkTheme = true;
    _loadFromPrefs();
  }

  toggleTheme() {
    _darkTheme = !_darkTheme;
    _saveToPrefs();
    notifyListeners();
  }

  _initPrefs() async {
    if (_prefs == null) _prefs = await SharedPreferences.getInstance();
  }

  _loadFromPrefs() async {
    await _initPrefs();
    _darkTheme = _prefs.getBool(key) ?? true;
    notifyListeners();
  }

  _saveToPrefs() async {
    await _initPrefs();
    _prefs.setBool(key, _darkTheme);
  }
}

For the dark theme, I followed this guide and maybe there is something wrong.