NullReferenceException in SQL Server 2014

I want to save checkboxlist data in a database. I created a table in SQL named 'tblSubject' and made connectionstring in web.config. However I still get the erorr :

NullReferenceException was unhandled by user code
object reference not set to an instance of an object.

This is the code in c#:

 private void PopulateSubjects()
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select * from subjects";
            cmd.Connection = conn;
            conn.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    ListItem item = new ListItem();
                    item.Text = sdr["Subject"].ToString();
                    item.Value = sdr["Id"].ToString();
                    item.Selected = Convert.ToBoolean(sdr["IsSelected"]);
                    chbox.Items.Add(item);
                }
            }
            conn.Close();
        }
    }
}

protected void Button1_Click(object sender, EventArgs e)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "update subjects set IsSelected = @IsSelected" +
                              " where Id=@Id";
            cmd.Connection = conn;
            conn.Open();
            foreach (ListItem item in chbox.Items)
            {
                cmd.Parameters.Clear();
                cmd.Parameters.AddWithValue("@IsSelected", item.Selected);
                cmd.Parameters.AddWithValue("@Id", item.Value);
                cmd.ExecuteNonQuery();
            }
            conn.Close();
        }
    }

}

And in Web.config:

 <connectionStrings>
  <add name=" constr" connectionString="Data Source=(LocalDB)\MSSQLLocalDB;
  Initial Catalog = dbtblSubject; Integrated Security = true" providerName="System.Data.SqlClient" />
 </connectionStrings>

Any help would be much appreciated.