Prevent InvalidArgumentException when source YAML is invalid

Problem/Motivation

When editing the form YAML source on the "Build" tab, invalid YAML may result in an InvalidArgumentException. For example, if one pastes the output of the "Export" tab instead of the source tab of another form, the error may be:

InvalidArgumentException: "uuid" is an invalid render array key. Value should be an array but got a string. in Drupal\Core\Render\Element::children() (line 97 of core/lib/Drupal/Core/Render/Element.php).

Proposed resolution

Use a try-catch block and set an error on the form component.

Remaining tasks

Implement.

User interface changes

Error message instead of exception for invalid YAML.

API changes

None.

Data model changes

None.