setState() challenge in Flutter

gistfile1.txt

Hello Devs,

I keep in receiving this error message "I/flutter (11828): Another exception was thrown: setState() called in constructor: CartState#040e6(lifecycle state: created, no widget, not mounted)
". I believe this is due to how I'm using setState() method in my CartState class below:

class CartScreen extends StatefulWidget{

  createState(){
    return CartState();
  }
}

class CartState extends State<CartScreen>{

  List cart = [];
  int counter = 1 ;

  void storeInCart(int id) {
    setState((){
      cart.add(id);
    });
    print("added successfully");
  }

  int getCart(){
    return cart.length;
  }

  Widget build(context){
    return Scaffold(
        appBar:  AppBar(
            title:  Center(
              child: Text('Shopping Cart'),
            ),
        ),
        body: Center(
          child: Text("Number of added of items is ${storeInCart(2)}")
        ),
    );
  }

}




import 'package:flutter/material.dart';
import 'login_screen.dart';
import 'checkout/checkout_screen.dart';
import 'login_screen.dart';
import '../models/store_model.dart';
import 'bottom_nav_screens/cart_screen.dart';

class ItemDetailScreen extends StatefulWidget {
  final String itemName;
  final double itemPrice;
  final String itemImage;
  final double itemRating;
  final int itemId;

  ItemDetailScreen(
      {this.itemId, this.itemName, this.itemPrice, this.itemImage, this.itemRating});

  createState() {
    return ItemDetailState();
  }
}


class ItemDetailState extends State<ItemDetailScreen> {
  .
  .
  .
  .
  Widget itemDescriptionWidget() {
    return Stack(
      children: <Widget>[
        .
        .
        .
        .
                Container(
                    child: Column(
                      children: <Widget>[
                        Row(
                          mainAxisAlignment: MainAxisAlignment.start,
                          children: <Widget>[
                            ButtonTheme(
                              minWidth: 195.0,
                              height: 50.0,
                              shape: RoundedRectangleBorder(borderRadius:BorderRadius.zero),
                              child: RaisedButton(
                                  child:  Text('Add to cart', style: TextStyle(color: Colors.white)),
                                  color: Colors.black,
                                  elevation: 4.0,
                                  splashColor: Colors.blueGrey,
                                  onPressed: () {
                                    CartScreen();
                                    CartState cart = CartState();
                                     cart.storeInCart(widget.itemId);
                                     _navCart(context);
                                  }),
                            ),
                            ButtonTheme(
                              minWidth: 195.0,
                              height: 50.0,
                              shape: RoundedRectangleBorder(borderRadius:BorderRadius.zero),
                              child: RaisedButton(
                                  child:  Text('Buy Now', style: TextStyle(color: Colors.white)),
                                  color: Colors.green,
                                  elevation: 4.0,
                                  splashColor: Colors.blueGrey,
                                  onPressed: () {
                                    if(login_screen.getLoginStatue() == true){
                                      return _navCheckoutScreen(context);
                                    }else{
                                      return _navLogin(context);
                                    }
                                  }),
                            )
                          ],
                        ),
                        Container(margin: EdgeInsets.only(bottom: 20.0)),

                        Row(
                          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                          children: <Widget>[
                            Icon(Icons.favorite),
                            Icon(Icons.share)
                          ],
                        ),
                        Container(margin: EdgeInsets.only(bottom: 5.0)),
                        Divider(),
                        Container(margin: EdgeInsets.only(bottom: 10.0)),

                      ],
                    )),
                description(),
                reviewsWidget(),
              ],
            ),
          ),
        ),
      ],
    );
  }



}