Sorry missed one line in the code which I pasted in my question/thread. This line is there in my method in system.
gr_ddlb_category->set_selection_table( it_selection_table = lt_ddlb ). <<<<======
READ TABLE lt_ddlb INTO ls_ddlb INDEX 1.
gv_initial_category = ls_ddlb-key.
ENDIF.
rv_valuehelp_descriptor = gr_ddlb_category.