Add check for entity

Closes #3427505

src/Plugin/Field/FieldFormatter/EntityReferenceExportFormatter.php

@@ -32,7 +32,7 @@ class EntityReferenceExportFormatter extends EntityReferenceEntityFormatter {
     $elements = $this->viewElements($items, $langcode);
 
     foreach ($items as $k => $item) {
-      if(empty($elements[$k])) {
+      if (empty($elements[$k]) && $item->entity) {
         $items->removeItem($k);
       }
     }