สอบถาม ปัญหาการ error Unable to cast object of type 'System.DBNull' to type 'System.String'. ครับ

                //Mysql Database
                objConn = new MySqlConnection();
                string strConnString = null;
                string strSQL = null;

                strConnString = "Server=localhost;User Id=root; Password=1234; Database=database;    Pooling=false";
                objConn.ConnectionString = strConnString;
                objConn.Open();

                strSQL = null;
                strSQL = "SELECT * FROM member ORDER BY DateTime DESC";

                
                objCmd = new MySqlCommand(strSQL, objConn);
                MySqlDataAdapter sda = new MySqlDataAdapter();
                sda.SelectCommand = objCmd;
                DataTable dt = new DataTable();
                sda.Fill(dt);
                GridView1.DataSource = dt;
                GridView1.DataBind();

                objConn.Close();
                objConn = null;