diff -u b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
--- b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
@@ -106,7 +106,9 @@
 
         // We have to prevent an endless loop when having two entities
         // referencing each other.
-        $parent_entity->recursiveValidationRunning = TRUE;
+        if ($parent_entity) {
+          $parent_entity->recursiveValidationRunning = TRUE;
+        }
 
         // The entities which could be created should run through the entity
         // validation before being saved.