How do I fix this strange null error

 string getOrder = "SELECT * FROM `wpet_woocommerce_order_items` a LEFT JOIN `wpet_woocommerce_order_itemmeta` b ON a.order_item_id = b.order_item_id LEFT JOIN `wpet_users` f ON f.ID = b.meta_value AND b.meta_key = 'seller_id' LEFT JOIN `wpet_dokan_orders` c  ON a.order_id = c.order_id LEFT JOIN `wpet_posts` d ON a.order_id = d.ID   WHERE a.order_id =" + cx.Field<ulong>("ID")  + " AND a.order_item_type = 'line_item' AND d.post_status = 'wc-completed';";
                    conn.Open();
                    MySqlCommand cmd = new MySqlCommand(getOrder, conn);

                    status.Text = conn.State.ToString();
                    // conn.Open();
                    MySqlDataAdapter dt = new MySqlDataAdapter(cmd);

                    DataTable wcOrder_data = new DataTable();
                    dt.Fill(wcOrder_data);

                    GridView3.DataSource = wcOrder_data;
                    GridView3.DataBind();

                    var toListing = wcOrder_data.Select().ToList();
                    // var reform = wcOrder_data.AsEnumerable();
                    conn.Close();

//ERROR POINT -->
                    var getUniqueItems = wcOrder_data.AsEnumerable().Where(x => x.Field<ulong>("order_item_id") > 0).GroupBy(x => x.Field<ulong>("order_item_id")).Select(g => new
                    {
                        ItemID = g.Key.ToString(),
                        ItemName = g.Select(x => x.Field<string>("order_item_name")).FirstOrDefault(),
                        Order = g.Select(x => x.Field<UInt64>("order_id").ToString()).FirstOrDefault(),
                        Vendor = g.Select(x => x.Field<Int64>("seller_id")).FirstOrDefault().ToString(),
                        ItemAmount = g.Where(x => x.Field<string>("meta_key") == "_line_subtotal").Select(x => x.Field<string>("meta_value").ToString()).FirstOrDefault(),
                        ItemLineTotal = g.Where(x => x.Field<string>("meta_key") == "_line_total").Select(x => x.Field<string>("meta_value").ToString()).FirstOrDefault(),

                        Commission = g.Where(x => x.Field<string>("meta_key") == "_dokan_commission_rate").Select(x => x.Field<string>("meta_value").ToString()).FirstOrDefault(),
                        Qty = g.Where(x => x.Field<string>("meta_key") == "_qty").Select(x => x.Field<string>("meta_value").ToString()).FirstOrDefault(),
                        CommissionToMall = double.Parse(g.Where(x => x.Field<string>("meta_key") == "_line_subtotal").Select(x => x.Field<string>("meta_value")).FirstOrDefault()) * (double.Parse(g.Where(x => x.Field<string>("meta_key") == "_dokan_commission_rate").Select(x => x.Field<string>("meta_value")).FirstOrDefault()) / 100),
                        OrderDate = g.Select(x => x.Field<DateTime>("post_date")).FirstOrDefault(),
                        SellerEmail = g.Select(x => x.Field<string>("user_email")).FirstOrDefault(),
                        VendorName = g.Select(x => x.Field<string>("display_name")).FirstOrDefault(),




                    });

ERROR: Value cannot be null. Parameter name: value::::::mscorlib::: at System.Number.ParseDouble(String value, NumberStyles options, NumberFormatInfo numfmt) at System.Double.Parse(String s) at App.tester.<>c.b__2_2(IGrouping`2 g) in C:\Users\Kent\Documents\Visual Studio 2015\Projects\App\tester.aspx.cs:line 60 at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() at App.tester.Button1_Click(Object sender, EventArgs e) in C:\Users\Kent\Documents\Visual Studio 2015\Projects\App\tester.aspx.cs:line 84