settings: fix setting_notify thread-safety

src/settings/setting/custom_setting.py

@@ -142,7 +142,10 @@ class CustomSetting(BaseSetting):
             _, notification, seconds = parts
 
             from ...main import get_main_window
-            get_main_window().setting_notify(
+            from gi.repository import GLib
+
+            GLib.idle_add(
+                get_main_window().setting_notify,
                 self.module.name,
                 self._(notification),
                 int(seconds) if seconds else None