Warning: array_flip(): Can only flip STRING and INTEGER values

Problem/Motivation

I've got a Drupal 8.6.4 install and added Nero theme.
The version is nero-8.x-1.4
Just after setting this theme as default, I've got the following warnings when reloading the front page:

Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php

I tried the same theme on a clean Drupal 8.6.4 install and same result.
The error is always reproductible.
The problem is severe because the theme is not usable as is.

Hope it'll be fixed and let me know if you need help.

Kind regards,
Vlad VALICA