Reference is lost when updating selection option

Question

Hi all,
In my application, I need to update selection screen according to user input, like update dropdown list. Therefore, I write my coding at view event WDDOMODIFYVIEW to ensure my method UpdateScreen is called at every roundtrip. Everything runs OK except when I click sign/option and choose any sign of selection option following dump occurs:
The following error text was processed in the system ICX : Access via 'NULL' object reference not possible.
The error occurred on the application server pwdf4646_ICX_12 and in the work process 0 .
The termination type was: RABAX_STATE
The ABAP call stack was:
Method: IF_WD_CONTEXT_ELEMENT~SET_ATTRIBUTE of program CL_WDR_CONTEXT_ELEMENT========CP
Method: SET_SIGN of program CL_WDR_SELECT_OPTIONS=========CP
Method: ON_SIGN_OPTION_LEAVE of program CL_WDR_SELECT_OPTIONS=========CP
Method: ONACTIONON_ENTRY_SELECTED of program /1BCWDY/ALMK9CLI947HVDFYUZV5==CP
Method: ONACTIONON_ENTRY_SELECTED of program /1BCWDY/ALMK9CLI947HVDFYUZV5==CP
Method: IF_WDR_VIEW_DELEGATE~WD_INVOKE_EVENT_HANDLER of program /1BCWDY/ALMK9CLI947HVDFYUZV5==CP
Method: INVOKE_EVENTHANDLER of program CL_WDR_DELEGATING_VIEW========CP
Method: IF_WDR_ACTION~FIRE of program CL_WDR_ACTION=================CP
Method: DO_HANDLE_ACTION_EVENT of program CL_WDR_WINDOW_PHASE_MODEL=====CP
Method: PROCESS_REQUEST of program CL_WDR_WINDOW_PHASE_MODEL=====CP
It losts ref since I call if_wd_select_options~upd_selection_field before choosing sign. If user clicks sign/option, there's no need to update selection screen because no user input changes. But how can I judge this?

Answer 1

Please let me comment more since no attachment can be uploaded. Following are key steps I do:
1) Build selection screen of two select options with this:
  lr_range = lr_selopt->create_range_table( i_typename = 'CRMT_PROCESS_TYPE' ).
  lr_selopt->add_selection_field(
    i_id                       = 'ACT_PROCTYPE'
    it_result                  = lr_range
    i_description              = 'Transaction Type'
  lr_range = lr_selopt->create_range_table( i_typename = 'CRMT_ACTIVITY_CATEGORY' ).
  lr_selopt->add_selection_field(
    i_id                          = 'ACT_CAT'
    it_result                     = lr_range
    i_value_help_structure_field  = 'CATEGORY'
2) At method WDDOMODIFYVIEW, I update the description for example:
  lr_range = lr_selopt->create_range_table( i_typename = 'CRMT_PROCESS_TYPE' ).
  lr_selopt->upd_selection_field(
    i_id                       = 'ACT_PROCTYPE'
    it_result                  = lr_range
    i_description              = 'New Transaction Type'
Then, if I click the sign button before select option and choose a sign, it gets dump.
I know the calling of upd_selection_field interferes something, but I have to update screen in order to complying with user input.