Textfield promotion to autocomplete should not leave a plain string in default_value

Problem/Motivation

GroupActionBase::buildConfigurationForm() upgrades the group_id element from a textfield to an entity_autocomplete for every modeler except bpmn_io, but it leaves #default_value as the raw stored string:

'#default_value' => $this->configuration['group_id'] ?? '',
'#required' => TRUE,
];
if (!self::isBpmnIoModeller()) {
  $form['group_id']['#type'] = 'entity_autocomplete';
  $form['group_id']['#target_type'] = 'group';

entity_autocomplete requires an entity object. Building the form therefore throws:

The #default_value property has to be an entity object or an array of entity objects.

Because group_id is #required, it is never empty on a configured action, so this fires whenever the form is built outside bpmn_io. The two cases that cannot work at all are:

• a token, for example [target_group], which is the documented way to make the action reusable. The field description itself says "This field supports tokens". An autocomplete element can never represent a token.
• a stale ID, where the group has since been deleted.

Steps to reproduce

1. Configure "Group: add content" with group_id set to a token, or to the ID of a group that is later deleted.
2. Build the action configuration form anywhere other than bpmn_io. Rendering it server side is enough, no browser needed.
3. The exception above is thrown.

Found while exporting an ECA model for the ECA Guide library: the model exports fine, but its config form is skipped and the published documentation silently loses that one form.

Proposed resolution

Only upgrade to entity_autocomplete when the stored value is empty or resolves to a group, and hand the element the loaded entity. GroupActionBase::loadGroup() already does the resolution, including UUIDs, and already returns NULL for a token because it token-replaces with clear => TRUE.

$default_group = $this->loadGroup();
if (($default_group !== NULL || ($this->configuration['group_id'] ?? '') === '') && !self::isBpmnIoModeller()) {
  $form['group_id']['#type'] = 'entity_autocomplete';
  $form['group_id']['#target_type'] = 'group';
  $form['group_id']['#default_value'] = $default_group;

The empty case keeps the autocomplete for normal authoring. A token or an unresolvable ID keeps the textfield, which is the only widget able to represent a token anyway. Patch attached, tested against 1.2.2.

Remaining tasks

Review the patch. entity_id needs the same treatment if it ever gets the same autocomplete upgrade.

User interface changes

None for values that already worked. A group_id holding a token, or an ID whose group no longer exists, now renders as a plain textfield instead of throwing.

API changes

None.

Data model changes

None.

A related observation

isBpmnIoModeller() makes a widget choice based on which modeler is in use. The practical effect is that the same action renders one way in bpmn_io and throws everywhere else, including during server-side rendering where there is no modeler at all. The patch above fixes the crash without touching that, but the widget choice arguably should not depend on the modeler.