Error after installing Civi Entity module
Part of the process that CiviCRM Entity uses to integrate is to read metadata on the civicrm API entities, and also for fields with options lists, it fetches those with an API call...
This API call :

$result = civicrm_api3('Relationship', 'getoptions', [
  'field' =&gt; "relationship_type_id",
]);


Is what is producing the warning...

I'm calling this a CiviCRM Core bug/issue..

there is an issue here, I'll update it:
https://lab.civicrm.org/dev/core/issues/268