EntityReferenceRevisionsItem::onChange() fatals when the computed entity property is NULL

Problem/Motivation

This is a narrow follow-up from #2706883. That older issue is closed, and the latest guidance there was to open a fresh issue/MR for the current 8.x-1.14 behavior.

EntityReferenceRevisionsItem::onChange('entity') keeps target_id and target_revision_id in sync. The target_id write already handles an unset/new target through the computed entity reference property:

$target_id = $property->isTargetNew() ? NULL : $property->getTargetIdentifier();

The sibling target_revision_id write still calls getRevisionId() unconditionally:

$property->getValue()->getRevisionId()

If the computed entity property is NULL, that fatals with:

Call to a member function getRevisionId() on null

Minimal trigger:

$item->set('entity', NULL);

Real-world trigger: Drupal core DefaultContent can currently pass NULL into an entity_reference_revisions field after an unresolved dependency lookup. I filed the DefaultContent/Graph-side issue separately as #3595546. Core should fail earlier, but ERR can still avoid turning a NULL clear into a fatal.

Steps to reproduce

1. Create an entity_reference_revisions field pointing at a revisionable target entity.
2. Populate one field item with a saved target entity.
3. Call $item->set('entity', NULL).

Expected: target_id and target_revision_id are both cleared.
Actual: target_revision_id sync calls getRevisionId() on NULL and fatals.

Proposed resolution

Mirror the existing target_id behavior for target_revision_id:

$target_revision_id = $property->getValue()?->getRevisionId();

Then write that value to target_revision_id.

Tests

Add kernel coverage to EntityReferenceRevisionsSaveTest:

• create an ERR field,
• assert target_id and target_revision_id are populated from a saved target,
• call $item->set('entity', NULL),
• assert both target properties are NULL.

Prepared with AI assistance.