Drop Down list in ALV display

Hai All

I got one requirement that i want to display drop down list for a column in alv display.

  LOOP AT lt_fcat ASSIGNING <lwa_fcat>.
    IF <lwa_fcat>-fieldname EQ 'INDCAT'.
      <lwa_fcat>-edit = 'X'.
    <lwa_fcat>-drdn_hndl = 1.
    ENDIF.
  ENDLOOP.
  REFRESH:gt_drpdwn1.
  LOOP AT it_makt  INTO wa_makt.
    ls_drpdwn-handle = 1.
    ls_drpdwn-value = wa_makt-indcat.
    APPEND ls_drpdwn TO gt_drpdwn1.
CALL METHOD ref_grid->refresh_table_display.
CALL FUNCTION 'GET_GLOBALS_FROM_SLVC_FULLSCR'
IMPORTING
e_grid = ref_grid.
CALL METHOD ref_grid->check_changed_data.
CALL METHOD ref_grid->set_drop_down_table
EXPORTING
it_drop_down = gt_drpdwn1.

i wrote this code but is going to dump showing error ref_grid points to null

Waiting for your replies....