Inline Entity Form has the support of ConfigEntityReference. Current widget works properly with selecting of the existing config entity, but produce an error in case of creation of a new item.
The reason for that is that current class waits for FieldableEntityInterface class, but gets example of ConfigEntity (in my case it is menu entity).
TypeError: Drupal\inline_entity_form\ReferenceUpgrader::upgradeEntityReferences(): Argument #1 ($entity) must be of type Drupal\Core\Entity\FieldableEntityInterface, Drupal\system\Entity\Menu given, called in /var/www/docroot/modules/contrib/inline_entity_form/src/WidgetSubmit.php on line 57 in Drupal\inline_entity_form\ReferenceUpgrader->upgradeEntityReferences() (line 46 of /var/www/docroot/modules/contrib/inline_entity_form/src/ReferenceUpgrader.php)
1. Create a config entity reference field.
2. Set inline entity form widget for the field.
3. Allow creation of a new reference item on the widget.
4. Try to create a new item and save the form.
Change code that saves reference. I'm not sure that it is enough to change only type of the variable. We also have to test carefully that ContentEntity reference is not broken.
Closed: outdated
3.0
Code
Here is a PR that fixes the issue:
https://git.drupalcode.org/project/inline_entity_form/-/merge_requests/8...
Work must be done in the original, reverted, issue.