When the previous engine name is unknown, switch to the next engine in menu.

bus/ibusimpl.c

 /* vim:set et sts=4: */
 /* ibus - The Input Bus
  * Copyright (C) 2008-2010 Peng Huang <shawn.p.huang@gmail.com>
  * Copyright (C) 2008-2010 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 958 matching lines @@
     if (!ibus->use_global_engine) {
         engine_name = (gchar *) g_object_get_data (G_OBJECT (context), "previous-engine-name");
     }
     else {
         if (!ibus->global_previous_engine_name) {
             ibus->global_previous_engine_name = bus_ibus_impl_load_global_previous_engine_name_from_config (ibus);
         }
         engine_name = ibus->global_previous_engine_name;
     }
 
+    /*
+     * If the previous engine name is not found, switch to the next engine
+     * in the menu. This behavior is better than doing nothing.
+     */
+    if (!engine_name) {
+        bus_ibus_impl_context_request_next_engine_in_menu(ibus, context);
+        return;
+    }
     _context_request_engine_cb (context, engine_name, ibus);
 }
 
 static void
 _global_engine_destroy_cb (BusEngineProxy *engine,
                            BusIBusImpl    *ibus)
 {
     if (ibus->global_engine != engine) {
         return;
     }
@@ skipping 1042 matching lines @@
         g_hash_table_unref (ibus->hotkey_to_engines_map);
     }
 
     ibus->engines_hotkey_profile = ibus_hotkey_profile_new();
     ibus->hotkey_to_engines_map =
         g_hash_table_new_full (NULL, NULL, NULL, (GDestroyNotify) g_list_free);
 
     g_list_foreach (ibus->register_engine_list, (GFunc) _add_engine_hotkey, ibus);
     g_list_foreach (ibus->engine_list, (GFunc) _add_engine_hotkey, ibus);
 }