Object reference not set to an instance of an object in gridview data source

Question

hi friends

i have a shopping cart that has a button for add to cart;
here , there are my codes for click event of the button:

 protected void AddToCart(object sender, ImageClickEventArgs e)
    {
        UserControl cart = (UserControl)Master.FindControl("cart1");
        DataGrid mydg = (DataGrid)cart.FindControl("dg");
        Label lbl = (Label)cart.FindControl("lblTotal");

        product = productName;
        DataTable objDT = (DataTable)Session["Cart"];

        foreach (DataRow objDR in objDT.Rows)
        {
            if (objDR["product"] == product)
            {
               // objDR["Quantity"] = int.Parse(objDR["Quantity"].ToString()) + int.Parse(txtQuantity.Text);
                objDR["Quantity"] = int.Parse(objDR["Quantity"].ToString()) + 1;
                blnMatch = true;
            }
        }
        if (blnMatch == false)
        {
            objDR = objDT.NewRow();
            objDR["Quantity"] = quantity;
            objDR["Product"] = productName;
            objDR["Cost"] = Decimal.Parse(productcost);
            objDT.Rows.Add(objDR);
        }
        Session["Cart"] = objDT;
        mydg.DataSource = objDT;
        mydg.DataBind();
        lbl.Text = "????" + GetItemTotal();
    }

 at pink line i get the error

 Object reference not set to an instance of an object.

how can i set that?