Undefined array key "title" in MenuSettingsConstraintValidator when saving a pending revision

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Problem/Motivation

When a node has a pending revision (for example a draft created with Content Moderation), saving the node form or triggering an AJAX request in it without having clicked "Add/Edit menu link" produces:

Warning: Undefined array key "title" in Drupal\menu_ui\Plugin\Validation\Constraint\MenuSettingsConstraintValidator->validate() (line 42 of core/modules/menu_ui/src/Plugin/Validation/Constraint/MenuSettingsConstraintValidator.php).
Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated

When errors are displayed, the warning is printed into the AJAX response, which then fails with "An AJAX HTTP error occurred ... StatusText: parsererror", for example when editing a paragraph.

The module adds core's menu_ui_node_builder as entity builder. That builder copies $form_state->getValue('menu') to $node->menu. Until the widget has been loaded, the menu form only contains menu[customize], so $node->menu holds no link values. MenuSettingsConstraintValidator runs for non-default revisions and reads $values['title'] without checking it.

The 2.x branch has the same problem in Hooks::formNodeFormAlter(). There, menu_ui_node_builder() is also deprecated in drupal:11.4.0.

Steps to reproduce

1. Install Content Moderation and enable the Editorial workflow for the Basic page content type.
2. Enable the asynchronous menu widget for Basic page, with at least one available menu.
3. Create and publish a Basic page.
4. Edit it and save it as Draft, so that the node has a pending revision.
5. Edit the latest revision and save it again without clicking "Add/Edit menu link".
6. The warning is shown or logged.

Proposed resolution

Replace menu_ui_node_builder with a module-specific entity builder that only sets $node->menu when the widget has been loaded ($form_state->get('menu_ui_async_widget')). When the property is left unset, the validator returns early, which matches core's own comment: "do not attempt to validate the menu settings since they are not being modified".

Remaining tasks

• Fix on 1.0.x.
• Port to 2.x as a method on Hooks.
• Consider a kernel or functional test covering a pending revision.

User interface changes

None.

API changes

New function menu_ui_async_widget_node_builder() (1.0.x).

Data model changes

None.