Index: wp-admin/widgets.php
===================================================================
--- wp-admin/widgets.php	(revision 19317)
+++ wp-admin/widgets.php	(working copy)
@@ -74,6 +74,8 @@
 			'before_title' => '',
 			'after_title' => '',
 		));
+	} elseif ( empty( $wp_registered_sidebars[ $sidebar_id ] ) ) {
+		unset( $sidebars_widgets[ $sidebar_id ] );
 	}
 }