Webform prepopulate on multivalue fields returns BadRequestHttpException: Input value "*FIELD_NAME*" contains a non-scalar value.

See attached patch

I am not able to replicate this issue using the attached webform.

Please provide more information on how to reproduce this issue.

Can you reproduce this issue using https;//simplytest.me?

We experienced the same issue. The patch in #2 worked for us.

Patch #2 solve for our us. Was occurring on name and advanced address composite fields.

URL is something like:
https://test.site/form?name%5Bfirst%5D=Tester&name%5Blast%5D=Test&address%5Bpostal_code%5D=10000-0000&address%5Bcountry_code%5D=US

This is due to a breaking change in Symfony 6 (used by Drupal 10) that no longer allows arrays as the return type of `InputBag::get`. See a similar fix that was made in another area of the Webform module: https://www.drupal.org/project/webform/issues/3335962 🐛 File input value contains a non-scalar value Fixed

This is a pretty high priority regression IMO, it will give an error and fail to render the webform on any link that is using prepopulated arrays in the query string.

@jrockowitz Attached is a repro webform. The difference here is that yours has the global `form_prepopulate` setting enabled, which does not trigger this error, as it follows a different codepath at `WebformSubmissionForm.php:L2651`. Disabling the global setting and enabling prepopulate on the individual form element allows the error to happen.

I'm having the same issue with prepopulating a custom composite field instantiated from a custom module that I created, and the patch resolves it. However, I can't reproduce the issue on a simplytest.me instance using a custom composite I created through the UI. There must be another factor involved.

@see 🐛 Prepopulate fails in Drupal 10 Needs review

@danekin I will give you commit credit on that ticket.