Asp.net how to correct the error
<p>im designing my web page, in that i have typed in the book id and typed
Nameofthebook.  and then click the button the  stack of book are available is go to another page and then not available is display the error message is book is not available My project (Library management system) </p>

<pre><code>     using System;
 using System.Collections;
 using System.Configuration;
 using System.Data;
 using System.Linq;
 using System.Web;
 using System.Web.Security;
 using System.Web.UI;
 using System.Web.UI.HtmlControls;
 using System.Web.UI.WebControls;
 using System.Web.UI.WebControls.WebParts;
 using System.Xml.Linq;
 using System.Data.SqlClient;

 public partial class Bookcheck : System.Web.UI.Page
 {

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string constr = null;
        constr = 





 ConfigurationManager.ConnectionStrings["librarymanagementconnetionstring"].ConnectionStrin

    g

  ;
        SqlConnection cnn = new SqlConnection(constr);
        cnn.Open();
        DataSet ds = new DataSet();
        DataTable dt = new DataTable();
        SqlDataAdapter da = new SqlDataAdapter("SELECT*FROM BOOKREGISTRATIONDETAILS WHERE   
     bookId='" + txtid.Text.Trim() + "'AND Nameofbook='" + txtnb.Text.Trim() + "'", 
     constr);
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count &gt; 0)
        {
            Response.Redirect("Issueofbook.aspx");
        }
        else
        {
            Msg.Text = "NO book available";
            Msg.Visible = true;
        }
    }
     }

 }




              My error
           NULL reference exception in un handle by the user

               constr =  




 ConfigurationManager.ConnectionStrings["librarymanagementconnetionstring"].ConnectionStrin
 g;
</code></pre>
