Exception databaseSystem.NullReferenceException in c#

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database

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Hello,
I have l'exception:
" System.NullReferenceException: Object reference not set to an instance of an object."
when I debug this code:

private void bt_valider_Click(object sender, System.EventArgs e)
        {
            OdbcConnection cn = new OdbcConnection("DSN=cp22");
            int rs1;
            try
            {
                cn.Open();
            }
            catch
            {
                MessageBox.Show("Failed to connect to data source");
            }
            finally
            {
                OdbcCommand comm;
                comm = new OdbcCommand("select * from utilisateur where login=?", cn);
                comm.Parameters.Add("login", login.Text);
                OdbcDataReader rs;
                try
                {
                    rs = comm.ExecuteReader();
                    if (rs.Read())
                    {
                        if (rs.GetString(1) == pass.Text)
                        {
                            rs.Close();
                            OdbcCommand cmd1 = new OdbcCommand("update utilisateur set login=?,mot_de_passe=?,niveau=? where login=?", cn);
                           // cmd1.Parameters.Add("login", login_new.Text)
                       cmd1.Parameters.Add( "mot_de_passe", pass_new.Text);
                            if (type_new.SelectedItem.ToString() == "")
                                cmd1.Parameters.Add("type", "u");
                            if (type_new.SelectedItem.ToString() == "Administrateur")
                                cmd1.Parameters.Add("type", "a");
                            if (type_new.SelectedItem.ToString() == "utilisateur")
                                cmd1.Parameters.Add("type", "u");
                            cmd1.Parameters.Add("", login.Text);
                            rs1 = cmd1.ExecuteNonQuery();
                            MessageBox.Show(this, "User modified", "", MessageBoxButtons.OK, MessageBoxIcon.Information);
                            login.Text = "";
                            pass.Text = "";
                            login_new.Text = "";
                            pass_new.Text = "";
                            
                        }
                        else
                            MessageBox.Show(this, "Invalid old password  ", "", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    }
                    else
                        MessageBox.Show(this, "User does not exist", "", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                catch (Exception eo)
                {
                   // MessageBox.Show(this, "Error System", "", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    login.Text = eo.ToString();
                }
            }
            cn.Close();
        }

Is anyone can help me please ant thnks for your reply.

the debugger stops in this line

rs = comm.ExecuteReader();