Asp.net how to solve 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"].ConnectionString;
        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;
        }
    }       
}      
</code></pre>

<p>Error:<br>
Object reference not set to an instance of an object.</p>
