open-uri: avoid criticals when app to open does not exist
open-uri: avoid criticals when app to open does not exist

If g_desktop_app_info_new() returns NULL, then the desired desktop file
does not exist. Simply return FALSE in this case instead of trying to
use it.

See: #480

matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal