Cannot submit Checkboxes / Radio / Dropdown field on a Form.

Steps:

1. Create new Form.
2. Add Checkboxes field.
3. Edit settings.
4. Fill Display label & Help.
5. Enter option name.
6. Submit.

Actual: Error 500 "Call to a member function getIdentifier() on null [in vendor/ezsystems/ezplatform-form-builder~/src/bundle/Controller/FieldController.php:168]"

Remarks: Affects also Radio and Dropdown.