Fix scene parser reading non-built-in scripts for POT generation

editor/plugins/packed_scene_translation_parser_plugin.cpp

@@ -80,6 +80,10 @@ Error PackedSceneEditorTranslationParserPlugin::parse_file(const String &p_path,
 			if (property_name == "script" && property_value.get_type() == Variant::OBJECT && !property_value.is_null()) {
 				// Parse built-in script.
 				Ref<Script> s = Object::cast_to<Script>(property_value);
+				if (!s->is_built_in()) {
+					continue;
+				}
+
 				String extension = s->get_language()->get_extension();
 				if (EditorTranslationParser::get_singleton()->can_parse(extension)) {
 					Vector<String> temp;