How to make a table field as read-only in a standard webdynpro component

Hi all - I am trying to make a screen table field as read-only in webdynpro component GRAC_UIBB_ACCESS_REQUEST view
V_REQUEST_ITEMS. Table N_ITEM_DATA has the field valid_from and that needs to be set as read only. I have tried below code in POST-EXIT on the method WDDOMODIFYVIEW. But it isn't working, ui_element is empty after this code is triggered. Please help, if anyone knows how to do this.

DATA : ui_element TYPE REF TO cl_wd_input_field.

ui_element ?= view->get_element( id = 'VALID_FROM' ).

ui_element->set_read_only(  abap_true ).

Thanks in advance.

Pranav