System.NullReferenceException: Object reference not set to an instance of an object.

I'm trying to upload a file using fileupload control and also getting some info in my textbox as input and converting the entire form data into xmldata.I'm geeting error as "System.NullReferenceException: Object reference not set to an instance of an
object."
your help is much appreciated on this regard.
my cs file
        protected void Button1_Click(object sender, EventArgs e)
        {
           
            if (FileUpload1.HasFile)
                try
                {
                    string filename ...