Ticket #27767: 27767-06.patch

src/wp-admin/js/customize-widgets.js

@@ -942 +942 @@
                         params.action = 'update-widget';
                         params.wp_customize = 'on';
                         params.nonce = api.Widgets.data.nonce;
+                        params.theme = api.settings.theme.stylesheet;
 
                         data = $.param( params );
                         $inputs = this._getInputs( $widgetContent );