TypeError if YAML contains custom tag

Problem/Motivation

YAML can contain custom tags: https://symfony.com/doc/current/components/yaml.html#parsing-and-dumping...
I don't think it is useful in webform but it should not break forms.

If the webform YAML contains a custom tag, it triggers this error:

TypeError:
strlen(): Argument #1 ($string) must be of type string, Symfony\Component\Yaml\Tag\TaggedValue given

Steps to reproduce

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

foo:
  '#markup': !my_tag
    foo: bar

Proposed resolution

The error could be handled more gracefully.

Remaining tasks

User interface changes

API changes

Data model changes