Hey guys, I have to reopen this, because suddenly after upgrade to 8.6.1 I have this issues as well.
It appeared after the bot visited robots.txt
Type php
Date Wednesday, December 12, 2018 - 03:21
User Anonymous (not verified)
Location https://xxxx/robots.txt
Referrer
TypeError: Argument 2 passed to Drupal\page_cache\StackMiddleware\PageCache::storeResponse() must be an instance of Symfony\Component\HttpFoundation\Response, null given, called in /home/dbacecde/xxx.com/core/modules/page_cache/src/StackMiddleware/PageCache.php on line 188 in Drupal\page_cache\StackMiddleware\PageCache->storeResponse() (line 205 of /home/dbacecde/xxx.com/core/modules/page_cache/src/StackMiddleware/PageCache.php) #0 /home/dbacecde/xxx.com/core/modules/page_cache/src/StackMiddleware/PageCache.php(188): Drupal\page_cache\StackMiddleware\PageCache->storeResponse(Object(Symfony\Component\HttpFoundation\Request), NULL) #1 /home/dbacecde/xxx.com/core/modules/page_cache/src/StackMiddleware/PageCache.php(121): Drupal\page_cache\StackMiddleware\PageCache->fetch(Object(Symfony\Component\HttpFoundation\Request), 1, true) #2 /home/dbacecde/xxx.com/core/modules/page_cache/src/StackMiddleware/PageCache.php(75): Drupal\page_cache\StackMiddleware\PageCache->lookup(Object(Symfony\Component\HttpFoundation\Request), 1, true) #3 /home/dbacecde/xxx.com/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #4 /home/dbacecde/xxx.com/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #5 /home/dbacecde/xxx.com/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #6 /home/dbacecde/xxx.com/core/lib/Drupal/Core/DrupalKernel.php(669): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #7 /home/dbacecde/xxx.com/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #8 {main}.