Fatal error when roles value is NULL in AccountForm::buildEntity()

Problem/Motivation

In Drupal\user\AccountForm::buildEntity(), the roles field is processed using the key() function without first checking whether the value retrieved from $form_state is an array. If the roles field is not present in the current form step (as is common in multistep registration forms), this causes a fatal error:

TypeError: key(): Argument #1 ($array) must be of type array, null given

This makes it impossible to use multistep registration or profile forms when the roles field is conditionally excluded or rendered in later steps.

Steps to reproduce

Install and enable the simple_multistep contributed module.

Proposed resolution

Modify the logic in AccountForm::buildEntity() to ensure key() is only called when the roles value is a valid array:

<?php
$roles = $form_state->getValue('roles');
if (is_array($roles) && is_string(key($roles))) {
  $form_state->setValue('roles', array_keys(array_filter($roles)));
}
?>

This prevents a fatal error when the roles field is not present in the form state.

Remaining tasks

Confirm the fix works in single-step and multi-step forms.
Create a test to validate behavior when is not set.
Review the patch/merge request once created.
Ensure no regressions in user role assignment functionality.

User interface changes

None.

Introduced terminology

None.

API changes

None. This is an internal fix within a core form method.

Data model changes

None.

Release notes snippet

Fixed a fatal error in user.module when the roles form value is missing during user registration or profile editing. This improves compatibility with multi-step or conditionally-rendered user forms.