Object of class Drupal\Core\Field\FieldItemList could not be converted to int

The file is permanent check in ImageStylesWarmer service in validateImage method is broken.

Notice: Object of class Drupal\Core\Field\FieldItemList could not be converted to int in Drupal\image_style_warmer\ImageStylesWarmer->validateImage() (line 135 of /image_style_warmer/src/ImageStylesWarmer.php)