Taxonomy Terms More than one level and Clean Class throws error
The error message:


  Recoverable fatal error: Object of class
  Drupal\Core\Field\FieldItemList could not be converted to string


Unfortunately the field object FieldItemList can't be casted to a string, it doesn't have a magic __toString() method, but it does have a magic __get() method. So you can provide a property name, which is for most field types value, to get the field value as string:


  class FieldItemList
  
  Represents an entity field; that is, a list of field item objects.
  
  An entity field is a list of field items, each containing a set of
  properties. Note that even single-valued entity fields are represented
  as list of field items, however for easy access to the contained item
  the entity field delegates __get() and __set() calls directly to the
  first item.


Source: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21FieldItemList.php/class/FieldItemList/8.7.x