Restarted drone...

I do not believe that this should be merged just on code review. We need to find out what changed in the code to cause this error first. If we don't find the cause we cannot be certain that this is the only place effected by that change or even if that change was correct. We have to stop just applying band-aid solutions and get to the root of the problem.

GRRR... When one does not know, one does not talk.
The error is obvious and if, again, instead of trying to demolish whatever some people do, you were looking at the code and tried to understand the issue, you would have just kept your mouth shut.
I suggest you look at wherever the fieldgroups type is used in core and you will see that the code I added IS necessary.
The line missing is just an error. The solution is not a bandaid. Period.

Please stop with the personal attacks and read what I wrote

Instead of adding new line, fix existing:

Well the field is name-spaced we either have to hard-code all the namespaces inside the JForm class

use Joomla\Component\Fields\Administrator\Field
...

• or we will use addfieldprefix="..." inside the XML files which will be used by JForm to load the correct field from the correct place

Commit
infograf768@ba02226#diff-419ad9989292871e19cda98184ab834a

added definition of appropriate namespace but
it only added it to the

<fields name="request"> ...</fields>

The fieldset that contains the field is not inside the <fields> , it is outside of it
and it should be changed also from

		<fieldset name="contact" label="COM_CONTACT_BASIC_OPTIONS_FIELDSET_LABEL"
			addfieldpath="/administrator/components/com_fields/models/fields"
			>

to

		<fieldset name="contact" label="COM_CONTACT_BASIC_OPTIONS_FIELDSET_LABEL"
			addfieldprefix="Joomla\Component\Fields\Administrator\Field"
			>

This PR is correct / needed, but better change the fieldset ?

@ggppdk solution is better as the addfieldpath is not used anymore in J4.

@ggppdk @laoneo

https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/components/com_contact/config.xml#L339-L348
https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/components/com_contact/forms/contact.xml#L663-L672
https://github.com/joomla/joomla-cms/blob/4.0-dev/components/com_contact/tmpl/contact/default.xml#L318-L327

As all fieldsgroups fields use directly addfieldprefix="Joomla\Component\Fields\Administrator\Field" in the field itself, I have preferred that solution over adding it in fieldset.

My idea was that if we need this only once, it is a better solution, as it shows precisely in the field itself where we need to load the field from instead of using it in the fieldset as we have here (and is anyway broken now in 4.0).

<fieldset name="contact" label="COM_CONTACT_BASIC_OPTIONS_FIELDSET_LABEL"
			addfieldpath="/administrator/components/com_fields/models/fields">

Here indeed is a good example as it is the only field needing the load in that fieldset

I agree we have a lot of remaining addfieldpath that should be taken care of, but this PR here is specific to solve an obvious bug and I suggest we take care of them in another PR where we would normalize this behavior.

What do you think?

Guess it wasn't such an obvious fix after all and your personal attack was exactly that

...

It doesn't mater if the addfieldprefix is added per field or on a fieldset, because it is added internally to the same scope.

@laoneo Then please merge. Then someone can make a patch for the addfieldpath

I have tested this item ✅ successfully on 1cc4136

Since i have tested this, i am posting a successful test

Before patch the error described (field is not found / not loaded),
after patch a select list with 'All' plus the field groups of users (not the field groups of contacts) appear

Better to replace the addfieldpath with addfieldprefix. It makes no sense to merge this one and then to revert it with the followup pr. Better to do it right from the beginning.

I would not have reverted it with the following PR. I would just have taken out the addfieldpath in that case, as I think we should do when only ONE field in the fieldset is concerned by the load See #21261 (comment)

Basically, the idea is that we should reserve adding in the fieldset when there are multiple type fields who need it in it.

Thanks.