[Dillo-dev] null URL

Looking at http://starurchin.org/dillo/valgrind/14566b8d334f46a5fa74873a6ea3f68e0d418aa... my guess is that something in nav stack cleaning made the const DilloUrl *url = a_History_get_url(NAV_TOP_UIDX(bw)); in a_UIcmd_page_popup() unhappy.