C# MySql 数据库代码出错“resultDs.Tables”引发了类型“System.NullReferenceException”的异常

Question

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

        private void SelectResultForm_Load(object sender, EventArgs e)
        {
            string sql = "select ca.id,c.name as coures_name,t.real_name as teacher_name,ca.course_time,ca.course_place,ca.max_num";
            sql += " from course_arrange ca,course c, teacher t where ca.course_id = c.id and ca.teacher_id = t.id";
            DataSet ds = SQLHelper.GetData(sql);
            courseArrangeGridView.DataSource = ds.Tables[0];
            int firstArrangeId = (int)ds.Tables[0].Rows[0][0];


            sql = "select * from select_result where course_arrange_id" + firstArrangeId;
            DataSet resultDs = SQLHelper.GetData(sql);
            ResultGridView.DataSource = resultDs.Tables[0];//出错
        }

        private void courseArrangeGridView_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            int selectId = (int)courseArrangeGridView.SelectedRows[0].Cells[0].Value;
            string sql = "select * from select_result where course_arrange_id" + selectId;
            DataSet resultDs = SQLHelper.GetData(sql);
            ResultGridView.DataSource = resultDs.Tables[0];//出错
        }

运行结果及报错内容

“System.NullReferenceException:未将对象引用设置到对象的实例“