when i'm using 3 tier insertion try-catch, showingstack overflow exception..i'm new to the try-catch block pls help me!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.Data;


namespace Wpf_threetier_test
{

   public class DAL
    {

       SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=DATABASE_THOTH;Integrated Security=True");
      SqlCommand cmd;



        public string  inset( BAL objbal)
        {

            try
            {

                con.Open();
                cmd = new SqlCommand();
                cmd.Connection = con;
                cmd.CommandText = "sp_test_db";
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@name", objbal.Name);
                cmd.Parameters.AddWithValue("@email", objbal.Email);
                cmd.Parameters.AddWithValue("@flag", "insert");
                cmd.ExecuteNonQuery();
                con.Close();
                string output = "saved in to the data base";
                return output;

            }
            catch(NoNullAllowedException x)
            {

                return null;
            }


            finally
            {
                con.Dispose();
            }




        }

    }
}