Проблема с подключением WPF к базе sql

System.Configuration.ConnectionStringSettingsCollection.this[string].get вернул null.
пытаюсь подключить базу к программе но при открытии выдаёт вот такую ошибку.


Вот код:
using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.Configuration;

using System.Data;

using System.Data.SqlClient;

using System.Diagnostics;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows ;

using System.Windows .Controls;

using System.Windows.Data;

using System.Windows .Documents;

using System.Windows .Input;

using System.Windows.Media ;

using System.Windows.Media .Imaging;

using System.Windows .Navigation;

using System.Windows .Shapes;



namespace UchetGSM

{

/// <summary>

/// Логика взаимодействия для PageDog.xaml

/// </summary>

public partial class PageDog : Page

{

string connectionString;

SqlDataAdapter adapter;

DataTable dogTable;



public PageDog()

{

InitializeComponent();

connectionString = ConfigurationManager.ConnectionStrings["УчетГСМConnectionString"].ConnectionString;

}



private void Page_Loaded(object sender, RoutedEventArgs e)

{

string sql = "SELECT * FROM Договора";

dogTable = new DataTable();

SqlConnection connection = null;

try

{

connection = new SqlConnection(connectionString);

SqlCommand command = new SqlCommand(sql, connection);

adapter = new SqlDataAdapter(command);

connection.Open ();

dogGrid.IsReadOnly = false;

adapter.Fill(dogTable);

dogGrid.ItemsSource = dogTable.DefaultView;



}

catch (Exception ex)

{

MessageBox.Show(ex.Message);

}

finally

{

if (connection != null)

connection.Close();

}

}

}

}