fix the issue of Error: Call to a member function isReversed

src/EntitySubqueueItemsFieldItemList.php

@@ -13,14 +13,18 @@ class EntitySubqueueItemsFieldItemList extends EntityReferenceFieldItemList {
    * {@inheritdoc}
    */
   public function appendItem($value = NULL) {
-    // If the queue is reversed, new items should be be added to the top of the
-    // queue.
-    if ($this->getEntity()->getQueue()->isReversed()) {
+    // Get the queue associated with the entity.
+    $queue = $this->getEntity()->getQueue();
+
+    // Check if the queue is available and is reversed.
+    if ($queue && $queue->isReversed()) {
+      // If the queue is reversed, new items should be added to the top of the queue.
       $item = $this->createItem(0, $value);
       array_unshift($this->list, $item);
       return $item;
     }
 
+    // If the queue is not reversed or not available, use parent implementation.
     return parent::appendItem($value);
   }