Contact form throws error on submitting

Problem/Motivation

Core contact form throws error when submitting form.

TypeError: Drupal\Component\Utility\Html::escape(): Argument #1 ($text) must be of type string, null given, called in /var/www/html/web/core/lib/Drupal/Component/Render/FormattableMarkup.php on line 238 in Drupal\Component\Utility\Html::escape() (line 431 of core/lib/Drupal/Component/Utility/Html.php).

Steps to reproduce

Drupal core 11.0.x, php 8.3.
Create contact form, subject field disabled.
Sending e-mail gives above error.

If Subject field is enabled, error goes away.

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet