The ConnectionString property is not initialized. How to fix?

Question

SQL

How to fix this message?
Everything works fine, until the moment when I want to write the changed datagrid data to the database

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!");

        }

Answer 1

Connection works for you only within using. It also closes it.
Or you need to create and store a SqlConnection for a long time, most likely all the time the program is running.
Or, in the ButtonSave_Click handler, recreate the SqlConnection and assign it to the adapter.Connection property.
Re-creating a SqlConnection in a short period of time does not lead to a physical re-creation of the database connection, since the provider still holds connections for some time after closing in case they are needed.