WM: always reset message-bus on file load

Previously this was only happening when "Load UI" was enabled,
making it difficult for Python script authors to know when
re-registering subscribers was needed.

source/blender/windowmanager/intern/wm_files.c

@@ -204,6 +204,16 @@ static void wm_window_match_init(bContext *C, ListBase *wmlist)
       WM_event_remove_handlers(C, &win->modalhandlers);
       ED_screen_exit(C, win, WM_window_get_active_screen(win));
     }
+
+    /* NOTE(@campbellbarton): Clear the message bus so it's always cleared on file load.
+     * Otherwise it's cleared when "Load UI" is set (see #USER_FILENOUI & #wm_close_and_free).
+     * However it's _not_ cleared when the UI is kept. This complicates use from add-ons
+     * which can re-register subscribers on file-load. To support this use case,
+     * it's best to have predictable behavior - always clear. */
+    if (wm->message_bus != NULL) {
+      WM_msgbus_destroy(wm->message_bus);
+      wm->message_bus = NULL;
+    }
   }
 
   /* reset active window */