Issue #3186745 by fago: User deprecated function "Setting slot value to an...

Issue #3186745 by fago: User deprecated function "Setting slot value to an array is not supported" when message is output

src/Processor/FileReferenceFieldItemListProcessor.php

@@ -26,7 +26,11 @@ class FileReferenceFieldItemListProcessor implements CustomElementProcessorInter
     assert($data instanceof FieldItemListInterface);
     $field_item_list = $data;
 
-    $element->setSlot($field_item_list->getName(), $field_item_list->view($viewMode), 'div', ['type' => $field_item_list->getFieldDefinition()->getType()]);
+    $nested_element = CustomElement::createFromRenderArray($field_item_list->view($viewMode))
+      ->setTagPrefix('field-')
+      ->setTag($field_item_list->getFieldDefinition()->getType());
+
+    $element->setSlotFromCustomElement($field_item_list->getName(), $nested_element);
   }
 
 }