Headers issue when dumping out variables in <theme>_preprocess_node

Issue #3104610 on drupal.org by mchaplin.

Trying to dump out variables in theme_preprocess_node:

kint($variables['node']);

Getting this error:

The website encountered an unexpected error. Please try again later.
RuntimeException: Failed to start the session because headers have already been sent by "/home/xxxxx/xxxxx-project/web/modules/contrib/devel/kint/kint/Kint.class.php" at line 263. in Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() (line 145 of /home/xxxx/xxxxx-project/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php).
Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() (Line: 164)
Drupal\Core\Session\SessionManager->startNow() (Line: 118)
Drupal\Core\Session\SessionManager->start() (Line: 57)
Symfony\Component\HttpFoundation\Session\Session->start() (Line: 240)
Drupal\big_pipe\Render\BigPipe->performPreSendTasks() (Line: 295)
Drupal\big_pipe\Render\BigPipe->sendContent(Object) (Line: 112)
Drupal\big_pipe\Render\BigPipeResponse->sendContent() (Line: 374)
Symfony\Component\HttpFoundation\Response->send() (Line: 20)