Simple Adding from datagridview

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C#3.5

I am working with an application and need to calculate the sum off all values of a column in data grid, just to show the user how much amount he spent on the job code when he selects it.
I called a data set and view all the data items and tried to add it but exception keeps on coming I tried all the ways I know its simple but not working

My field to be added is Long integer in database and I fill the table using dataset
and after that I called the add function.

Please can anyone help. Don't avoid hurting my feelings simple because I am a beginner

My code

public DriverPaymentForm(DriverPaymentBean driverpaymentbean)
        {
            InitializeComponent();

            fillForm(driverpaymentbean);
            showallPaymentbyjobcode();
            addtotalpaid();

        }

public void showallPaymentbyjobcode()
        {
            try
            {



                String query = "Select jobcode ,vehicleno,amount,postdate,description,ischeque,iscash from driverpaymentmastertable  where jobcode Like '" + txtjobCode.Text + "'  Order by postdate";
                OleDbDataAdapter dAdapter = new OleDbDataAdapter(query, connString);
                DataSet ds = new DataSet();
                dAdapter.Fill(ds, "tblpayview");
                tblpaymentview.DataSource = ds.Tables["tblpayview"].DefaultView;
                if (ds.Tables.Count <= 0)
                {
                    lblstatus.Text = "No Payment Details Present";
                }
                
            }
            catch (Exception)
            {
                MessageBox.Show("The application had met with some errors please restart  the application :\n error:closer MSAccess files");

            }


        }

        public void addtotalpaid()
        {
           long  sum = 0;
            int count = tblpaymentview.RowCount;
            if (count != null&count!=0)
            {

                for (int i = 0; i < count; i++)
                {

                    sum = sum + long.Parse(tblpaymentview.Rows[i].Cells[3].Value.ToString());

                }
            }

        }

Exception null reference exception at addition line