Getting error as Object reference not set to an instance of an object.

Hi,
I am implementing the pagination for the gridview to view the records in the gridview row. But what happening here is I am getting error as
Object reference not set to an instance of an object.. Please see the code, I am not able to find out why I am getting the below error:-
private DataView getDataForPagination() { using (SqlConnection conn = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["DefaultCSRConnection"].ConnectionString)) { DataSet ds = new DataSet(); string strSelectedCmd = "Select ID, name, email,class,attendance,english_subject_marks,math_subject_marks,academic_performance,extra_activities,social_skills,general_health,date_of_record,modified_date,status,active from tbl_student_report"; SqlDataAdapter da = new SqlDataAdapter(strSelectedCmd, conn); ...