IIRC, dot notation returns an any value. So PB is trying to do an implicit cast of a null any value to a long any value. But by definition null has no datatype so PB can't figure out the casting. So why not cast it yourself?
of_testingnullarg( Long( tab_2.tabpage_comp_info.dw_webcomp_info.object.dormant[i] ) )
Wheeley