\Drupal\entity_reference_revisions\Plugin\Field\FieldFormatter\EntityReferenceRevisionsFormatterBase::prepareView doesn't protect against unloadable entities

Yeah i could easily imagine this to happen if ERR is used for non-paragraphs use case - such as pointing to node targets.
For Paragraphs, it's not a release blocker, because there is no UI to list and delete the paragraphs.

Maybe this Error-Log could help you:

The website encountered an unexpected error. Please try again later.
TypeError: Argument 1 passed to Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase::checkAccess() must implement interface Drupal\Core\Entity\EntityInterface, null given, called in /www/htdocs/w01418d9/drupal8/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php on line 55 in Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase->checkAccess() (line 182 of core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php).
Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase->checkAccess(NULL) (Line: 55)
Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase->getEntitiesToView(Object, 'de') (Line: 131)
Drupal\entity_reference_revisions\Plugin\Field\FieldFormatter\EntityReferenceRevisionsEntityFormatter->viewElements(Object, 'de') (Line: 80)
Drupal\Core\Field\FormatterBase->view(Object, 'de') (Line: 76)
Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase->view(Object, NULL) (Line: 259)
Drupal\Core\Entity\Entity\EntityViewDisplay->buildMultiple(Array) (Line: 303)
Drupal\Core\Entity\EntityViewBuilder->buildComponents(Array, Array, Array, 'vorschaubild') (Line: 24)
Drupal\node\NodeViewBuilder->buildComponents(Array, Array, Array, 'vorschaubild') (Line: 246)
Drupal\Core\Entity\EntityViewBuilder->buildMultiple(Array) (Line: 203)
Drupal\Core\Entity\EntityViewBuilder->build(Array)
call_user_func(Array, Array) (Line: 381)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 226)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 227)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 117)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 62)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 98)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 77)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 628)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

+++ b/src/Plugin/Field/FieldFormatter/EntityReferenceRevisionsFormatterBase.php
@@ -19,7 +19,15 @@ abstract class EntityReferenceRevisionsFormatterBase extends EntityReferenceForm
     foreach ($entities_items as $items) {
       foreach ($items as $item) {
-        $item->_loaded = TRUE;
+
+        if ($this->needsEntityLoad($item)) {
+          $target_type = $this->getFieldSetting('target_type');
+          $entity = \Drupal::entityTypeManager()->getStorage($target_type)->loadRevision($item->target_revision_id);
+          if ($entity) {
+            $item->entity = $entity;
+            $item->_loaded = TRUE;
+          }

The change was done on purpose, see the comment above.

Doing this breaks things like previewing paragraphs. Looks like we have no tests here, but I'm sure paragraph tests would fail with this change.

If we need something like this, then I think we can simply do this:
if ($item->entity) {
$item->_loaded = TRUE;
}

We can only do single-load anyway. We should slightly update the comment above then, to say way access it to trigger lazy loading and making sure it exists.

+++ b/src/Plugin/Field/FieldFormatter/EntityReferenceRevisionsFormatterBase.php
--- /dev/null
+++ b/tests/modules/entity_composite_relationship_test/Entity_composite_relationship_test.permissions.yml

+++ b/tests/src/Kernel/EntityReferenceRevisionsFormatterTest.php
@@ -0,0 +1,106 @@
+
+    $user = $this->createUser(['administer entity_test composite relationship']);
+    \Drupal::currentUser()->setAccount($user);

The test fails for me as well, with phpunit and run-tests.sh:

1) Drupal\Tests\entity_reference_revisions\Kernel\EntityReferenceRevisionsFormatterTest::testFormatterWithDeletedReference
Invalid permission administer entity_test composite relationship.

The reason it's not failing for you is because OSX is stupid :)

tests/modules/entity_composite_relationship_test/Entity_composite_relationship_test.permissions.yml

Can you spot the error?

I just tested the last patch with latest Paragraphs dev and it breaks ParagraphsPreviewTest, it seems that in preview some Paragraphs are not displayed even if they have content.

@dawehner: No, you misunderstood. Paragraphs tests pass right now. This change breaks them.

It might not be your use case, but we need to find a way to fix this without breaking something that works right now.

Discussed with @Berdir, this change shouldn't break Paragraphs either. Just to know, in Paragraphs we were testing that preview is shown when Paragraphs are not saved yet, the change was made to avoid the check for new entities.

Unrelated test fail.

Yay.

Test fail fix: #2770705: Fix test fails related with diff.

Fixed the related head fail first, retested, committed. Thx all! :-)