settings: fix handle_callback thread-safety

src/settings/setting/custom_setting.py

@@ -126,7 +126,9 @@ class CustomSetting(BaseSetting):
     def _handle_callback(self, line):
         try:
             _, action, target, value = line.split(':', 3)
-            self.section.handle_callback(
+            from gi.repository import GLib
+            GLib.idle_add(
+                self.section.handle_callback,
                 action.strip(),
                 target.strip(),
                 value.strip()