"Failed to start the session because headers have already been sent" - while indexing

Problem/Motivation

While indexing a page I get an error "RuntimeException while trying to render item entity:node/10006:de with view mode search_index for search index Index A: Failed to start the session because headers have already been sent by "/app/vendor/symfony/http-foundation/Response.php" at line 1285. in Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() (line 116 of /app/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php)."

The stack trace indicates this starts in CookiePreviewDetector::isPreviewing method.

#0 /app/docroot/core/lib/Drupal/Core/Session/SessionManager.php(129): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start()
#1 /app/docroot/core/lib/Drupal/Core/Session/SessionManager.php(94): Drupal\Core\Session\SessionManager->startNow()
#2 /app/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php(282): Drupal\Core\Session\SessionManager->start()
#3 /app/vendor/symfony/http-foundation/Session/Session.php(201): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->getBag('attributes')
#4 /app/vendor/symfony/http-foundation/Session/Session.php(221): Symfony\Component\HttpFoundation\Session\Session->getBag('attributes')
#5 /app/vendor/symfony/http-foundation/Session/Session.php(69): Symfony\Component\HttpFoundation\Session\Session->getAttributeBag()
#6 /app/docroot/modules/contrib/entity_reference_preview/src/Plugin/PreviewDetector/CookiePreviewDetector.php(51): Symfony\Component\HttpFoundation\Session\Session->get('entity_referenc...')
#7 /app/docroot/modules/contrib/entity_reference_preview/src/Cache/StatusPreviewCacheContext.php(68): Drupal\entity_reference_preview\Plugin\PreviewDetector\CookiePreviewDetector->isPreviewing(Object(Symfony\Component\HttpFoundation\Request))
#8 [internal function]: Drupal\entity_reference_preview\Cache\StatusPreviewCacheContext->Drupal\entit

Steps to reproduce

Set up Search API.
Enable this module.
Used the rendered output plugin in your search index.
Check the logs.

Proposed resolution

We should check that the session is actually started before trying to access the cookie. That gets rid of the error for me. If it hasn't just return FALSE. I don't think returning FALSE really matters in search index because you only want to render default revision content.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A