nemo-blank-desktop-window.c: Fix potential crash when actions

are updates.

Desktop windows can be destroyed when monitors power off or
configurations changed. The blank desktop windows were not being
disconnected from the correct action manager when disposed, causing
invalid callbacks later.

Fixes #3571

Author: mtwebster

src/nemo-blank-desktop-window.c

@@ -240,11 +240,13 @@ nemo_blank_desktop_window_dispose (GObject *obj)
     g_clear_handle_id (&window->details->actions_changed_idle_id, g_source_remove);
 
     if (window->details->actions_changed_id > 0) {
-        g_signal_handler_disconnect (nemo_desktop_manager_get_action_manager (),
+        g_signal_handler_disconnect (window->details->action_manager,
                              window->details->actions_changed_id);
         window->details->actions_changed_id = 0;
     }
 
+    g_clear_object (&window->details->action_manager);
+
     clear_ui (window);
 
     g_signal_handlers_disconnect_by_func (nemo_menu_config_preferences, reset_popup_menu, window);