Undefined property: CRM_Contact_DAO_Contact::$contact_id in civicrm_entity_contact_user_get()
The message is a 'Notice' which can sometimes be ignored.  However, looking at the code causing the message the purpose of the function is to load a user entity based on the contact id but the contact id is not set which is fundamental to the purpose of that function.

It would take a bit of debugging to track down where and why that is being called from your rule in a way that causes the contact_id not to be set - and ideally you would do that.  Alternatively providing it is does what you need in creating the Drupal user correctly you could choose to ignore it.