There is already an open DataReader associated with this Command which must be c

i am want to search record from two columns ... from one text box and one button .. i am getting this eror 

There is already an open DataReader associated with this Command which must be c

 protected void Button1_Click(object sender, EventArgs e)
        {

try
            {
            con.Open();
            SqlCommand cmd = new SqlCommand( "SELECT * FROM KSMCemp_data WHERE National_ID =" + TextBox1.Text  , con);
            SqlDataAdapter da1 = new SqlDataAdapter(cmd);
            SqlDataReader Dr = cmd.ExecuteReader();
            DataSet ds = new DataSet();

            if (Dr.Read())
            {
                da1.Fill(ds);
                GridView2.DataSource = ds;
                GridView2.DataBind();
                con.Close();
                Dr.Close();
            }

            else
            {
                con.Close();
                con.Open();
                DataSet ds2 = new DataSet();
                SqlCommand cmnd = new SqlCommand("SELECT * FROM KSMCemp_data WHERE serial_no =" + TextBox1.Text, con);
                SqlDataAdapter da2 = new SqlDataAdapter(cmnd);
                SqlDataReader Dr1 = cmd.ExecuteReader();

                if (Dr1.Read())
                {
                    da2.Fill(ds2);
                    GridView2.DataSource = ds2;
                    GridView2.DataBind();
                    con.Close();
                    Dr1.Close();
                }
                else
                {
                    Message("Reference ID Not Found", this);
                }


            }
            }
            catch
            {
                Message("ID Not Found", this);
            }

        }