LateInitializationError: Field 'productItem' has not been initialized

late means you are telling Flutter that i will make sure that this field gets initialized, don't do null checks on this variable. But you are using late and not initializing productItem, which is resulting in runtime error

You need to start productItem variable. When you use late, you guarantee that you will start right after this variable.

Use

Product product = Product(id, name);

Instead of

late Product product;

or

late Product product;
product = Product(id, name);

Edit:

I'm adding below the code I put in the comment link of this answer and which @Anesu said worked.

class ProductItem {
  String id;
  String bottleName;
  String imgUrl;
  String price;
  Bottle bottle;

  ProductItem({required this.id, required this.bottleName, required this.imgUrl, required this.price});
}

class ProductItemPage extends StatefulWidget {
  ProductItem item;
  
  ProductItemPage(
      {Key? key,
      required this.item})
      : super(key: key);

  @override
  State<ProductItemPage> createState() => _ProductItemPageState();
}

class _ProductItemPageState extends State<ProductItemPage>
    with AutomaticKeepAliveClientMixin {
  
   late ProductItem productItem;
  
  @override
  void initState() {
    super.initState();
    productItem = widget.item;
  }
  
  @override
  bool get wantKeepAlive => true;
  @override
  Widget build(BuildContext context) {
    super.build(context);
    var cart = Provider.of<ShoppingCartProvider>(context);
    var favoriteProvider = Provider.of<FavoriteProvider>(context);
    return Card(
      shadowColor: Colors.grey,
      surfaceTintColor: Colors.amber,
      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(20)),
      child: Stack(
        children: [
          Positioned(
            right: 0,
            child: InkWell(
              onTap: () {
                favoriteProvider.toggleFavorites(productItem.bottle);
                if (favoriteProvider.isExist(productItem.bottle)) {
                  ScaffoldMessenger.of(context).hideCurrentSnackBar();
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(
                      content: Text(
                        "Product Added to Favorite!",
                        style: TextStyle(fontSize: 16),
                      ),
                      backgroundColor: Colors.green,
                      duration: Duration(seconds: 1),
                    ),
                  );
                } else {
                  ScaffoldMessenger.of(context).hideCurrentSnackBar();
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(
                      content: Text(
                        "Product Removed from Favorite!",
                        style: TextStyle(fontSize: 16),
                      ),
                      backgroundColor: Colors.red,
                      duration: Duration(seconds: 1),
                    ),
                  );
                }
              },
              child: favoriteProvider.isExist(productItem.bottle)
                  ? Icon(
                      Icons.favorite,
                      color: Colors.redAccent,
                    )
                  : Icon(Icons.favorite_border),
            ),
          ),
          Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Center(
                child: Image.asset(
                  productItem.imgUrl,
                  height: 200.0,
                ),
              ),
              Center(
                  child: Text(productItem.bottle,
                      style: const TextStyle(
                          fontSize: 20.0, fontWeight: FontWeight.bold))),
              Center(
                child: Text('R${productItem.price}'),
              )
            ],
          ),
          Positioned(
              bottom: 0,
              right: 10,
              child: IconButton(
                icon: const Icon(Icons.add_circle),
                iconSize: 40.0,
                onPressed: () {
                  cart.addToCart(productItem.id, productItem.bottleName,
                      productItem.price, productItem.imgUrl);
                },
              ))
        ],
      ),
    );
  }
}