fix: use right index for the transition in SimpleFormIterator

[alanpoulain]

Before:

After:

It's probably because the id of the fields is changing (id is generated from useFieldArray), and the ref needs to be changed as well (see http://reactcommunity.org/react-transition-group/transition/#Transition-prop-nodeRef).

Using the previous code from master is needed to solve this issue.
I needed to revert #7123 as well, I don't understand how the change in this PR can work.

[WiXSL]

@alanpoulain, thanks for working on this.
It seems to work, although, I see some other problems like adding items after removing all items creates empty items.
But that is a different issue.

UPDATE:
If I remove the TransitionGroup/CSSTransition component completely, everything works.
So, the problem seems to be there, but I cannot find the issue.

[djhi]

Thanks!

[djhi]

I should have remembered why I used react-hook-form id instead of our own. This introduces weird issues that you can see on the deployed simple example for the PR.

Open https://react-admin-az7z4sx9x-marmelab.vercel.app/#/posts/13

Add two backlink, remove one, add another:

[alanpoulain]

Maybe the alternative is to make sure the id from useFieldArray doesn't change then.