SQL Connection Error: Connection property has not been initialized.

Hello sir ,

     i m getting following Error While Established SQL connection. i use window application c#.i m getting Following Error : connection property has not been initialized . please help. Here is my Code. i also Highlight part where i m Getting this Error. 

string connection = ConfigurationSettings.AppSettings["constrg"];
            SqlConnection con = new SqlConnection(connection);
            SqlDataAdapter sda = new SqlDataAdapter("select Count(*) From Login_tbl where User_name = '" +textBox1.Text+"' and Password = '"+textBox2.Text+"'",con);
            DataTable dt = new DataTable();
            sda.Fill(dt);
            if (dt.Rows[0][0].ToString() == "1")
            {
                this.Hide();
                Home mainwindow = new Home(textBox1.Text);
                mainwindow.Show();
            }

i m also provide my App.Config code Below : 

 <connectionStrings>
      <add name="constrg"
   connectionString="Data Source=.;Initial Catalog=almajad-smsdatabase;AttachDbFilename=|DataDirectory|\almajad-smsdatabase.mdf;Integrated Security=True"
   providerName="System.Data.SqlClient" />
    </connectionStrings>