Any idea how to pass list of seller names coming from database to a dropdown menu?

The get seller from database function:

late String getsellernames = http://ip/sas.php;

  var allSellerNames = [];

  getSas() async {
    var response = await http.get(Uri.parse(getsellernames));
    if (response.statusCode == 200) {
      setState(() {
        allSellerNames = json.decode(response.body);
      });
    }

    print('$allSellerNames');
    return allSellerNames;
  }

The dropdown

DropdownButtonHideUnderline(
                  child: DropdownButton(
                    // hint: const Text('Seller'),
                    value: allSellerNames[index][0],
                    items: allSellerNames[index]['seller'].map((ditem) {
                      return DropdownMenuItem(
                        child: Text(
                          ditem,
                        ),
                        value: ditem,
                      );
                    }).toList(),
                    onChanged: (String? dValue) {
                      setState(() {
                        initialnames = dValue!;
                      });
                    },
                  ),
                ),

But I got this error:
The argument type ‘void Function(String?)’ can’t be assigned to the parameter type ‘void Function(Object?)?’