why does a nullreference exception comes in passing parameters to a function that is specified in a dll file

See more:

C#

I have a clsSqlDBOperation.dll file added as reference in my application. it contains database based operation's functions like storedProcedureDataTable(). In his function parameters are passed from my form1's report_click event that is simple button.
Now while adding a storedProcedureDataTable() in this click event then adding its result in another dataTable then NullReference exception occurs.
Below is my code....

 DBOperation.clsSQLDBOperation objDB;


DataTable dtWfReport = new DataTable();

 ArrayList arrParam = new ArrayList();
                ArrayList arrValue = new ArrayList();
                TimePeriod();
                arrParam.Add("REPORTTYPE");
                arrValue.Add(1);
                arrParam.Add("STARTDATE");
                arrValue.Add(strStartdt);
                arrParam.Add("ENDDATE");
                arrValue.Add(strEndDate);
                arrParam.Add("USERID");
                arrValue.Add(listBoxControl1.SelectedValue);

               //while debugging problem comes over the code below
                dtWfReport = objDB.storeProcedureDataTable("WF_REPORTS", arrParam, arrValue);
                for (int i = 0; i <= dtWfReport.Rows.Count - 1; i++)
                {
                    if (string.IsNullOrEmpty(dtWfReport.Rows[i].ItemArray[1].ToString()))
                    {
                        dtWfReport.Rows[i].ItemArray[1] = "check ur entries";
                        break;
                    }
                }


                //objDB.closeConnection();



                //BindChart();

                
            }