In this commit 004b56a5c31841bc0bf4a9f17902bbff8d0f509d we added the `l10n_fr_pdp.what_is_pdp` action as PDP version of the `account_peppol.what_is_peppol` action.

But one case does not work / was not tested properly: We are registered on PDP already.
In that case we just want to go back to the "move send" wizard. But the `what_is_pdp` action does not support that currently. So there is a traceback.

Steps to reproduce:
1. Install `l10n_fr_pdp`
2. Activate French E-Invoicing / PDP in Demo mode
3. Create and post an invoice for a French PDP partner (e.g. just use the "FR Company")
4. Click "Send"
5. In the "move send" wizard disable the "French E-Invoicing (Demo)" option
6. Click on "Why should you use it?" in the warning
7. Click "Got it" in the window that pops up.
8. Traceback (see below)

```
Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/http.py", line 2167, in _transactioning
    return service_model.retrying(func, env=self.env)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/odoo/service/model.py", line 157, in retrying
    result = func()
             ^^^^^^
  File "/home/odoo/src/odoo/odoo/http.py", line 2134, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/odoo/http.py", line 2382, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/odoo/addons/base/models/ir_http.py", line 333, in _dispatch
    result = endpoint(**request.params)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/odoo/http.py", line 754, in route_wrapper
    result = endpoint(self, *args, **params_ok)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/addons/web/controllers/dataset.py", line 36, in call_kw
    return call_kw(request.env[model], method, args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/odoo/api.py", line 535, in call_kw
    result = getattr(recs, name)(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/odoo/src/odoo/addons/account_peppol/models/res_config_settings.py", line 159, in button_peppol_reregister
    self.ensure_one()
  File "/home/odoo/src/odoo/odoo/models.py", line 6277, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: res.config.settings()
```

task-None


Thanks to KYMAG for finding / reporting the issue.

Forward-Port-Of: odoo/odoo#282406