Null reference Exception

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Sample
{
    public partial class Fileupload : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
           
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            cl1 cl = new cl1();
            
            if (FileUpload1.HasFile)
                
               cl.check_save();     
            else
            {
                Label1.Text = "You have not specified a file";
            }
                
        }
        
            
        }

}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Sample;

namespace Sample
{
    public class cl1 : Fileupload
    {
        public void check_save()
        {
            int conlen = FileUpload1.PostedFile.ContentLength;
            if (conlen != null && conlen != 0)
            {
                if (FileUpload1.PostedFile.ContentLength > 400000)
                {
                    Label1.Text = "Please select a low size file";
                }
                else
                {
                    try
                    {

                        FileUpload1.SaveAs("...................." + FileUpload1.FileName);
                        Label1.Text = "Filename:" + FileUpload1.PostedFile.FileName + "<br>" + "Content type:" + FileUpload1.PostedFile.ContentType;
                    }
                    catch (Exception ex)
                    {
                        Label1.Text = "Error" + ex.Message.ToString();
                    }

                }
            }
        }

    }
}