Return of NULL value in Zendesk.php

Problem/Motivation

"PHP Fatal error: Uncaught TypeError: Return value of Drupal\\helpdesk_zendesk\\Plugin\\HelpdeskIntegration\\Zendesk::pushUser() must be of the type string, null returned in /var/www/drupal/kriens/develop/web/modules/contrib/helpdesk_zendesk/src/Plugin/HelpdeskIntegration/Zendesk.php:378", "Stack trace:", "#0 /var/www/drupal/kriens/develop/web/modules/contrib/helpdesk_integration/src/PluginBase.php(509): Drupal\\helpdesk_zendesk\\Plugin\\HelpdeskIntegration\\Zendesk->pushUser()", "#1 /var/www/drupal/kriens/develop/web/modules/contrib/helpdesk_integration/src/Service.php(300): Drupal\\helpdesk_integration\\PluginBase->updateAllUsers()", "#2 /var/www/drupal/kriens/develop/web/modules/contrib/helpdesk_integration/src/Service.php(337): Drupal\\helpdesk_integration\\Service->updateAll()", "#3 /var/www/drupal/kriens/develop/web/modules/contrib/bs_inventory/src/Rebuild.php(147): Drupal\\helpdesk_integration\\Service->sync()", "#4 [internal function]: Drupal\\bs_inventory\\Rebuild->execute()", "#5 {main}", " thrown in /var/www/drupal/kriens/develop/web/modules/contrib/helpdesk_zendesk/src/Plugin/HelpdeskIntegration/Zendesk.php on line 378"]