Object reference not set to an instance of an object.

Question

I keep getting ObJect not an instance of an object error on the 4th line.
which is (_selecteedAppointmentRow = (DataSetNumber1.TBL_AppointmentsRow) row.Row;

Here is the whole code. 

 if (appointmentWithEngineer != null)
            {
                row = (System.Data.DataRowView)appointmentWithEngineer.DataItem;
               _selectedAppointmentRow = (DataSetNumber1.TBL_AppointmentsRow)row.Row;
                if (_selectedAppointmentRow == null)
                    MessageBox.Show("Appointment is null");
}
}


Any help will be appreciated.


Regards