InvalidArgumentException if YAML contains invalid render array

Problem/Motivation

When editing a form without webform_uid, if the YAML input contains an invalid render array, it triggers an exception:

InvalidArgumentException:
"foo" is an invalid render array key. Value should be an array but got a object.

  at core/lib/Drupal/Core/Render/Element.php:97
  at Drupal\Core\Render\Element::children()
     (modules/contrib/webform/src/WebformEntityElementsForm.php:234)
  at Drupal\webform\WebformEntityElementsForm->addWebformTypePrefixRecursive()
     (modules/contrib/webform/src/WebformEntityElementsForm.php:216)

Steps to reproduce

Don't enable webform_ui.
Browse to /admin/structure/webform/manage/[webform_id].
Save this YAML:

foo: bar

Proposed resolution

This error should be handled more gracefully.

Remaining tasks

User interface changes

API changes

Data model changes