how do I run the sp that has an update from asp.net?

I have the following problem: I have an application in 3 layers in the data layer I have the following code:

 public string D_RegistroAprobacionRH(int vopt, string vObservacionVrh, string vUsuarioValidarh, string vObservacionArh, string vUsuarioApruebarh, string vObservacionLogistica, string vUsuarioApruebaLogist)
  {
      try
      {
          string _bol = "";

          sqlconx = new SqlConnection();
          sqlconx.ConnectionString = vConec.conxWebProd;

          dt = new DataTable();
          dAdap = new SqlDataAdapter();
          cmd = new SqlCommand();

          cmd.Connection = sqlconx;
          cmd.CommandTimeout = 7000000;
          cmd.CommandType = CommandType.StoredProcedure;
          cmd.CommandText = "Usp_RegitroValidacion";

          cmd.Parameters.Add(new SqlParameter("@Opt", SqlDbType.Int)).Value = vopt;
          cmd.Parameters.Add(new SqlParameter("@Observacionvrh", SqlDbType.VarChar,150)).Value = vObservacionVrh;
          cmd.Parameters.Add(new SqlParameter("@Usuariovrh", SqlDbType.VarChar, 30)).Value = vUsuarioValidarh;
          cmd.Parameters.Add(new SqlParameter("@ObservacionArh", SqlDbType.VarChar, 150)).Value = vObservacionArh;
          cmd.Parameters.Add(new SqlParameter("@UsuarioArh", SqlDbType.VarChar, 30)).Value = vUsuarioApruebarh;
          cmd.Parameters.Add(new SqlParameter("@ObservaLogi", SqlDbType.VarChar, 150)).Value = vObservacionLogistica;
          cmd.Parameters.Add(new SqlParameter("@UsuarioAprlog", SqlDbType.VarChar, 30)).Value = vUsuarioApruebaLogist;

          dt.Clear();
          dt.Reset();

          if (sqlconx.State == ConnectionState.Closed)
          {
              sqlconx.Open();

          }

         cmd.ExecuteNonQuery();

         sqlconx.Close();

          return _bol ;

      }
      catch (Exception ex)
      {

          throw new Exception(ex.Message);
      }

in the business layer I have the following code:

 public string B_RegistraAprobacionrh(int vopt, string vObservacionVrh, string vUsuarioValidarh, string vObservacionArh, string vUsuarioApruebarh, string vObservacionLogistica, string vUsuarioApruebaLogist)
   {
       return vDregistraValida.D_RegistroAprobacionRH(vopt, vObservacionVrh, vUsuarioValidarh, vObservacionArh, vUsuarioApruebarh, vObservacionLogistica, vUsuarioApruebaLogist);

   }

In the presentation I have the following code which is where I get an error:

protected void btnaprobarrh_Click(object sender, EventArgs e)
    {
        string Exec = "";

        Exec= vB_ApruebaRRHH.B_RegistraAprobacionrh(2, "", "", "", Session["Usuario"].ToString(), "", "");
        btnaprobarrh.Enabled = false;
        btnDescargar.Enabled = false;



        Response.Write("<script language=javascript>alert('Aprobación Registrada Correctamente...!');</script>");

    }

shows me the following message: Object reference not set to an instance of an object.