Add new item button is not working due to serialization problem

Here's some more call stack:

array_map(Array, Array) (Line: 60)
Drupal\Core\KeyValueStore\DatabaseStorageExpirable->getMultiple(Array) (Line: 40)
Drupal\Core\KeyValueStore\StorageBase->get('form-jbdHafjOYhsQV58CTuq0QAU3vj04YJB_E8En4UEnhDw') (Line: 125)
Drupal\Core\Form\FormCache->getCache('form-jbdHafjOYhsQV58CTuq0QAU3vj04YJB_E8En4UEnhDw', Object) (Line: 437)
Drupal\Core\Form\FormBuilder->getCache('form-jbdHafjOYhsQV58CTuq0QAU3vj04YJB_E8En4UEnhDw', Object) (Line: 262)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 79)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 128)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 577)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 129)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 102)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array) (Line: 139)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 62)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 62)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 103)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 82)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 55)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 631)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

And looking at what it is trying to serialize.. it looks like it is happening in the cache operation for the form, presumably during the Ajax call.

I did a web search, and found this:
https://bugs.php.net/bug.php?id=30791
which indicates that the error string saying that __sleep is not working is due to the fact that if you try to serialize an object with a magic __call() method on it, it also needs to have __sleep() defined, or the serialize will fail.

See also
http://php.net/manual/en/language.oop5.magic.php#object.sleep
http://php.net/manual/en/language.oop5.overloading.php#object.call -- especially comment #2

So yes, any class you are serializing that defines __call() also needs to define __sleep().

So I went looking in Core for classes that define __call() but not __sleep(). There are quite a few... the most possibly relevant ones are:
vendor/guzzlehttp/guzzle/src/Client.php
core/lib/Drupal/Core/Extension/Extension.php
core/lib/Drupal/Component/Datetime/DateTimePlus.php
vendor/doctrine/annotations/lib/Doctrine/Common/Annotations/IndexedReader.php
core/lib/Drupal/Core/Database/Query/SelectExtender.php
vendor/phpspec/prophecy/src/Prophecy/Prophecy/ObjectProphecy.php
vendor/symfony/debug/Exception/FlattenException.php
vendor/doctrine/common/lib/Doctrine/Common/Persistence/PersistentObject.php
core/lib/Drupal/Core/Plugin/Discovery/InfoHookDecorator.php

There are also a bunch in
vendor/zendframework/zend-feed
but I don't think they're relevant to this probably.

Looks like either this is a Core but or something in one of our vendor projects. Sigh.

OK, found two related issues.

Ah, this one is also related, because the Help Topic entity has a plugin collection.

However, that fix went in for 8.0.x so there is something else going on... the HelpTopic entity does have a __sleep() method that it gets from ConfigEntityBase:
https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Config!Entity!Con...

Hm. So the error I'm getting is:

__sleep should return an array only containing the names of instance-variables to serialize.

It looks like 4 config entities have overridden the __sleep() method that ConfigEntityBase provides... maybe this entity will need to do that too, since it looks like they were overridden for similar reasons. I think #1977206: Default serialization of ConfigEntities is trying to do that in a generic way, but who knows when that will be fixed. So. Probably need to fix it locally.

I was able to resolve this critical bug by adding serialize/unserialize methods to my config entity, similar to the ones in the patch for #1977206: Default serialization of ConfigEntities.

This has resurfaced and needs investigation again. Seems this time to just be an unserialize problem.

This time I should definitely add a serialize/unserialize test to the HelpTopicTest, I think, so that it hopefully doesn't happen again.

Well, OK. The problem seems to have vanished again -- perhaps I just needed to clear the cache. Anyway, I made a serialization test and committed it against this issue.