Why do I keep getting this error? I am getting this error message in the logs after installing RNG module for Drupal 8 and trying to submit a completed a event form. The error log contains this error.


  Uncaught PHP Exception Drupal\Core\Entity\EntityStorageException: "Value is not a valid entity." at /home/vhosts/***********/public_html/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php line 757, referer: http://**********/node/add/event_1


Any idea on how I can resolve this please?
I eventually traced back the issue as being caused by the rules module not RNG. Removing the rules enabled me to save the event.