This crash part of this ought to be fixed by making error_dialog
handle a NULL path.

The underlying cause can, I think, be fixed by...

1. Apply the gtkfilechooserdefault.c part of the latest patch from
   http://bugzilla.gnome.org/show_bug.cgi?id=136185.  (That makes
   it stop using get_file_info for verifying directories, in
   particular the root directory for which it doesn't work.)

2. Adding the following to gtk_file_system_unix_get_folder (which
   will make check_is_folder from 1 actually work):

      if (!g_file_test (filename, G_FILE_TEST_IS_DIR))
        {
          int save_errno = errno;
          gchar *filename_utf8 = g_filename_to_utf8 (filename, -1,
NULL, NULL, N
ULL);
          g_set_error (error,
                       GTK_FILE_SYSTEM_ERROR,
                       GTK_FILE_SYSTEM_ERROR_NONEXISTENT,
                       _("file is not a directory '%s': %s"),
                       filename_utf8 ? filename_utf8 : "???",
                       g_strerror (save_errno));
          g_free (filename_utf8);
          return NULL;
        }

    This part should, IMHO, be done in all cases.

*** Bug 137123 has been marked as a duplicate of this bug. ***

This was trigger by the trailing-slash patch.  I fixed that and the
crash.  (2) above was fixed also, (1) not.