Ticket #35928: 35928.patch

src/wp-includes/widgets.php

@@ -830 +830 @@
         global $wp_registered_widgets, $wp_registered_sidebars;
         $sidebars_widgets = get_option('sidebars_widgets');
         foreach ( (array) $wp_registered_sidebars as $index => $sidebar ) {
-                if ( count($sidebars_widgets[$index]) ) {
+                if ( isset( $sidebars_widgets[ $index ] ) && count( $sidebars_widgets[ $index ] ) ) {
                         foreach ( (array) $sidebars_widgets[$index] as $widget )
                                 if ( array_key_exists($widget, $wp_registered_widgets) )
                                         return true;