Incompatibility with Webform 6.0.0

Problem/Motivation

When upgrading from Webform 5 to Webform 6 (in this case from 8.x-5.20 to 6.0.0-alpha17), existing handlers using Mautic Webform cease working. PHP errors are thrown when trying to edit the Mautic Webform handler on any webform or when trying to add the handler to a new webform. Selecting to add or edit a Mautic Webform handler in the Webform interface fails to load the handler UI.

When trying to add or edit a Mautic Webform handler to any form, the following PHP error is thrown:

[01-Oct-2020 16:28:20 Australia/Brisbane] Error: Call to a member function get() on null in /path/to/site/docroot/modules/contrib/webform/src/Plugin/WebformHandlerBase.php on line 293 #0 /path/to/site/docroot/modules/contrib/webform/src/Form/WebformHandlerAddForm.php(37): Drupal\webform\Plugin\WebformHandlerBase->isExcluded()
#1 [internal function]: Drupal\webform\Form\WebformHandlerAddForm->buildForm(Array, Object(Drupal\Core\Form\FormState), Object(Drupal\webform\Entity\Webform), 'mautic')
#2 /path/to/site/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(532): call_user_func_array(Array, Array)
#3 /path/to/site/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(278): Drupal\Core\Form\FormBuilder->retrieveForm('webform_handler...', Object(Drupal\Core\Form\FormState))
#4 /path/to/site/docroot/core/lib/Drupal/Core/Controller/FormController.php(91): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\webform\Form\WebformHandlerAddForm), Object(Drupal\Core\Form\FormState))
#5 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch))
#6 /path/to/site/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#7 /path/to/site/docroot/core/lib/Drupal/Core/Render/Renderer.php(573): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#8 /path/to/site/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#9 /path/to/site/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#10 /path/to/site/vendor/symfony/http-kernel/HttpKernel.php(151): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#11 /path/to/site/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#12 /path/to/site/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#13 /path/to/site/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#14 /path/to/site/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#15 /path/to/site/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#16 /path/to/site/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /path/to/site/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /path/to/site/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /path/to/site/docroot/core/lib/Drupal/Core/DrupalKernel.php(708): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /path/to/site/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#21 {main}

Steps to reproduce

1. Upgrade from Webform 5 to Webform 6 (with Webform Mautic already installed).
2. Navigate to any existing Webform using a Webform Mautic handler and click 'Edit' on the handler.
3. See that clicking 'Edit' no longer has the intended effect and check logs for the above error.
4. Note that submissions are no longer passed to Mautic from any existing forms using a Webform Mautic handler.

Proposed resolution

The issue appears to lie in WebformMauticHandler.php as it contains code that has been changed and/or deprecated in Webform 6.

The solution appears to be updating WebformMauticHandler.php to replace the changed and/or deprecated code with code that is supported by Webform 6. Looking to the latest updates to Webform's RemotePostWebformHandler.php may be of assistance, as WebformMauticHandler.php appears to be based on an older version of that handler.

As the Varbase project recently moved from using Webform 5 to Webform 6, this issue may be affecting a large number of installations previously integrated with Mautic.