object refrence not set to object

DirectoryInfo directory = new DirectoryInfo(Server.MapPath("~/Booksimg/pdf/"));
          int counter = 0;
          foreach (FileInfo file in directory.GetFiles())
          {
              HyperLink link = new HyperLink();
              link.ID = "Link" + counter++;
              link.Text = file.Name;
              link.NavigateUrl = "Download.aspx?name=" + file.Name;

              //    Page.Controls.Add(link);
              //    Page.Controls.Add(new LiteralControl("<br/>"));
          }
      }
      protected void LinkButton10_Click(object sender, EventArgs e)
      {

          Response.Redirect("Download.aspx");
      }

protected void Page_Load(object sender, EventArgs e)
       {
           object us = Session["UserID"];
           if (Session["UserID"] == null || Session["RoleId"] == null)
               Response.Redirect("Login.aspx");
           else
           {
               user = Session["UserID"].ToString();
               String role = Session["RoleId"].ToString();


               cn.Open();
               string cmdstr = "SELECT unm from register where email='"+ user + "' ";
               SqlCommand cmd = new SqlCommand(cmdstr, cn);

               SqlDataReader rd = cmd.ExecuteReader();
               //if (rd.Read())
               //{

               //    Label2.Text = rd.GetString(1);

               //}
               rd.Close();



               string fileName = Request.QueryString["name"].ToString();// here is the error.

               Response.ContentType = "application/octet-stream";
               Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
               Response.TransmitFile(Server.MapPath("~/Booksimg/pdf/" + fileName));
               Response.End();
           }

       }