check that the underlying entity has the parent reference field before trying to get() it (#979)

Co-authored-by: Willow Gillingham <willow@born-digital.com>

src/Plugin/Condition/NodeHasParent.php

@@ -137,9 +137,9 @@ class NodeHasParent extends ConditionPluginBase implements ContainerFactoryPlugi
    *   TRUE if entity references the specified parent.
    */
   protected function evaluateEntity(EntityInterface $entity) {
-    foreach ($entity->referencedEntities() as $referenced_entity) {
-      if ($entity->getEntityTypeID() == 'node' && $referenced_entity->getEntityTypeId() == 'node') {
     $parent_reference_field = $this->configuration['parent_reference_field'];
+    foreach ($entity->referencedEntities() as $referenced_entity) {
+      if ($entity->getEntityTypeID() == 'node' && $referenced_entity->getEntityTypeId() == 'node' && $entity->hasField($parent_reference_field)) {
         $field = $entity->get($parent_reference_field);
         if (!$field->isEmpty()) {
           $nids = $field->getValue();