I can't reproduce either, even if the path does not exist, it adds
g_ptr_array_add (arr, g_strdup (g_get_tmp_dir ()));
g_ptr_array_add (arr, g_strdup (g_get_home_dir ()));
to the list, and on /tmp I can always write to.
--
crash if temp_iso_dir doesn't exist
https://launchpad.net/bugs/48231