Issue #3255637: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated

core/lib/Drupal/Component/Render/FormattableMarkup.php

@@ -259,7 +259,7 @@ protected static function placeholderFormat($string, array $args) {
    *   The properly escaped replacement value.
    */
   protected static function placeholderEscape($value) {
-    return $value instanceof MarkupInterface ? (string) $value : Html::escape($value);
+    return $value instanceof MarkupInterface ? (string) $value : Html::escape($value ?? '');
   }
 
 }