Object reference not set to an instance of an object.

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ASP.NET

I have created connection string in configuration file

and then created connection object to access that but it is giving error as follows

Object reference not set to an instance of an object.

What I have tried:

Connecton String is as

<connectionStrings>
  <add name="=ConStr" connectionString="Data Source=DESKTOP-HL3PK63; Database=ASP_Database; User Id=sa; Password=123" providerName="System.Data.SqlClient"/>
</connectionStrings>

.cs file code

public partial class WebForm1 : System.Web.UI.Page
    {
        SqlConnection cn = null;
        SqlCommand cmd = null;
        SqlDataAdapter da = null;
        DataSet ds = null;
        string strsqlCommand = string.Empty;
        protected void Page_Load(object sender, EventArgs e)
        {
            cn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConStr"].ConnectionString);
            if (!Page.IsPostBack)
            {
                BindRepeaterData();
            }

        }
 protected void BindRepeaterData()
        {
            strsqlCommand = "Select * from Comment Order By PostedDate Desc";
            da = new SqlDataAdapter(strsqlCommand, cn);
            ds = new DataSet();
            da.Fill(ds);
            Repeater1.DataSource = ds;
            Repeater1.DataBind();
        }