Issue #3241300 by alexpott: template_preprocess_responsive_image_formatter()...

Issue #3241300 by alexpott: template_preprocess_responsive_image_formatter() causes deprecations on PHP 8.1

core/modules/responsive_image/responsive_image.module

@@ -106,7 +106,7 @@ function template_preprocess_responsive_image_formatter(&$variables) {
   $item = $variables['item'];
   $attributes = [];
   // Do not output an empty 'title' attribute.
-  if (mb_strlen($item->title) != 0) {
+  if (!is_null($item->title) && mb_strlen($item->title) != 0) {
     $attributes['title'] = $item->title;
   }
   $attributes['alt'] = $item->alt;