Add Index to build Method where alertDialog is in Flutter

Hello I need to call the index to recuperate user information into an AlertDialog The build Method should have index but I dont know how to add. this is the variable need to call:

final Message chat = chats[index];

and I need index into build method:

class InfluencerDescription extends StatefulWidget {
  final User user;

  InfluencerDescription({this.user});

  @override
  _InfluencerDescriptionState createState() => _InfluencerDescriptionState();
}

class _InfluencerDescriptionState extends State<InfluencerDescription> {

  @override
  Widget build(BuildContext context ) {
    return AlertDialog(
      elevation: 20,
      content: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Expanded(
            child: SizedBox(
            ),
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              RaisedButton(
                  elevation: 10,
                  color: Colors.green,
                  child: Container(
                    width: 100,
                    height: 60,
                    child: Center(
                      child: Text(
                        'AVVIA UNA \n CHAT CON ME',
                        textAlign: TextAlign.center,
                        style: TextStyle(color: Colors.white,fontSize: 13),
                      ),
                    ),
                  ),
                  onPressed: () {
                    Navigator.push(
                      context,
                      MaterialPageRoute(
                        builder: (_) => ChatScreen(user: HERE NEED TO CALL CHAT.SENDER VAR,),),);
                  }
              ),