Can't Pass Value Between Pages Flutter

I need to pass "index" from my ListView to second page, i use this code

this is to pass

onTap: (){
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) => beforeMainCheckList(
                      index: index,
                    )
                    )
                );
          },

this is to get index

class beforeMainCheckList extends StatefulWidget {

 
  beforeMainCheckList({Key? key, required this.index  }) : super(key: key);

  final int index;

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

this is error that i got when i try to use index inside scaffold

Undefined name 'index'. Try correcting the name to one that is defined, or defining the name

You are using a StatefulWidget which means you can access the Widget's fields by using the widget variable and then the field name like that:

widget.index