i cant add function to onchanged The argument type 'void Function(Language)' can't be assigned to the parameter type 'void Function(Language?)?'

I’m trying to make an Internationalizing app with this youtube movie:
https://www.youtube.com/watch?v=yX0nNHz1sFo&list=PLyHn8N5MSsgEfPAxCytQDPATDlHwpP5rE&index=4

When I add a function to DropDownButton’s onChanged I get the error:
The argument type ‘void Function(Language)’ can’t be assigned to the parameter type ‘void Function(Language?)?’

Here is the code

class GeneralList extends StatefulWidget {
  GeneralList({super.key});

  @override
  State<GeneralList> createState() => _GeneralListState();
}

class _GeneralListState extends State<GeneralList> {
 void  _changeLanguage(Language language) {
    print(language);
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: DropdownButton2(
           onChanged: (Language language) {
            _changeLanguage(language);
          },
          items: Language.languageList()
              .map<DropdownMenuItem<Language>>(
                (lang) => DropdownMenuItem(
                  value: lang,
                  child: Row(
                    children: <Widget>[Text(lang.name)],
                  ),
                ),
              )
              .toList()),
    );
  }
}

I looked up for an answer in this post on stack:
Can't add a function to onChanged property

but I still get the same error

>Solution :

you need to make Language nullable

onChanged: (Language? language) {
  if (language != null) {
    _changeLanguage(language);
  }
},