Steps to reproduce

1. Use Ibexa Experience 4.6.4
2. Create a custom page block, see: https://doc.ibexa.co/en/latest/content_management/pages/create_custom_page_block/
3. In block configuration set an invalid value (e.g. embedx) for one of the attribute type
4. Edit a landing page and add a new block

Actual Result
The POST request /admin/page/block/request-configuration-form fails with a non-obvious exception

Neither the property "value" nor one of the methods "value()", "getvalue()"/"isvalue()"/"hasvalue()" or "__call()" exist and have public access in class "Symfony\Component\Form\FormView".

Expected result
At some point, there should be a proper error message that embedx is no valid attribute type.