🇫🇷France dxx
The field_group module only require the field_group_accordion if you use the accordion or accordion_item anywhere in your configuration.
Since your are updated your configuration (display views) to remove any usage of format_type: accordion or format_type: accordion_item, you can safety do the following to remove this deprecated modules (if you no longer use `jquery_ui`):
drush pmu jquery_ui
drush pmu jquery_ui_accordion
composer remove drupal/jquery_ui_accordion drupal/jquery_ui
💬 | Drupal core | Error: Call to a member function isNew() on null in Drupal\Core\Entity\Plugin\Validation\Constraint\ValidReferenceConstraintValidator->validate()
🇫🇷France dxx
If your field is not a reference to another entity, check that your field ItemList class extends the FieldItemList and not the EntityReferenceFieldItemList.
Also, in the same class, check your computeValue() method to ensure that the createItem() method is called with the right value (eg not an object).