Свойство ConnectionString не инициализировано. Как исправить?

static string stringConnect = ConfigurationManager.ConnectionStrings["OrdersConnection"].ConnectionString; 
SqlDataAdapter adapter;
DataSet dataSet;

public void ViewOrders()
        {
            string sqlCommandSelect = @"SELECT * FROM [ORDERS]";

            using (SqlConnection connection = new SqlConnection(stringConnect))
            {

                adapter = new SqlDataAdapter(sqlCommandSelect, connection);
                dataSet = new DataSet();
                adapter.Fill(dataSet, "ORDERS");

                gridForControl.ItemsSource = new DataView(dataSet.Tables["ORDERS"]);
            }
        }

        private void ButtonSave_Click(object sender, RoutedEventArgs e)
        {
            SqlCommandBuilder builder = new SqlCommandBuilder(adapter);
            adapter.Update(dataSet.Tables["ORDERS"]); // тут ошибка
            //ViewOrders();
            MessageBox.Show("Данные успешно обновленны!");

        }