Problem/Motivation
We're seeing the exception AssertionError: assert(is_string($chunk) || $chunk instanceof HtmlResponse) in assert() (line 259 of /var/www/html/web/core/modules/big_pipe/src/Render/BigPipe.php) with the following stack trace:
#0 /var/www/html/web/core/modules/big_pipe/src/Render/BigPipe.php(259): assert()
#1 /var/www/html/web/core/modules/big_pipe/src/Render/BigPipe.php(369): Drupal\big_pipe\Render\BigPipe->sendChunk()
#2 /var/www/html/web/core/modules/big_pipe/src/Render/BigPipe.php(304): Drupal\big_pipe\Render\BigPipe->sendPreBody()
#3 /var/www/html/web/core/modules/big_pipe/src/Render/BigPipeResponse.php(112): Drupal\big_pipe\Render\BigPipe->sendContent()
#4 /var/www/html/vendor/symfony/http-foundation/Response.php(381): Drupal\big_pipe\Render\BigPipeResponse->sendContent()
#5 /var/www/html/web/index.php(20): Symfony\Component\HttpFoundation\Response->send()
#6 {main}
This is caused by
[$pre_scripts_bottom, $scripts_bottom, $post_scripts_bottom] = explode('<drupal-big-pipe-scripts-bottom-marker>', $pre_body, 3);
// ...
$this->sendChunk($scripts_bottom);
in \Drupal\big_pipe\Render\BigPipe::sendPreBody where obviously the separator used by explode isn't present in the pre body string and hence, $scripts_bottom is NULL.
This is confirmed because of 2 warnings prior to the exception that tell us Warning: Undefined array key 1 and Warning: Undefined array key 2.
Steps to reproduce
I can't reproduce but I found out the context what must have happened: an unpublished node gets requested by an IP which comes from MailChimp. I suspect, that somebody at the customer site is preparing a newsletter and wants to refer to a node which is present but not published yet. And then, the MailChimp platform seems to undertake some fancy requests that lead to such exceptions.
Proposed resolution
Before calling for explode, we should test if <drupal-big-pipe-scripts-bottom-marker> is contained in the string and if not, just return without doing anything.
