On upgrading from 4.0.0-alpha1 to 4.0.0-alpha2 I get
The website encountered an unexpected error. Please try again later.
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "conditional_fields.element_alter_helper". Did you mean this: "conditional_fields.form_helper"? in Drupal\Component\DependencyInjection\Container->get() (line 157 of core\lib\Drupal\Component\DependencyInjection\Container.php).
Drupal::service('conditional_fields.element_alter_helper') (Line: 101)
conditional_fields_element_after_build(Array, Object)
call_user_func_array('conditional_fields_element_after_build', Array) (Line: 1081)
Drupal\Core\Form\FormBuilder->doBuildForm('views_exposed_form', Array, Object) (Line: 1073)
Drupal\Core\Form\FormBuilder->doBuildForm('views_exposed_form', Array, Object) (Line: 577)
Drupal\Core\Form\FormBuilder->processForm('views_exposed_form', Array, Object) (Line: 323)
Drupal\Core\Form\FormBuilder->buildForm('\Drupal\views\Form\ViewsExposedForm', Object) (Line: 152)
Drupal\views\Plugin\views\exposed_form\ExposedFormPluginBase->renderExposedForm() (Line: 1243)
Drupal\views\ViewExecutable->build() (Line: 392)
Drupal\views\Plugin\views\display\PathPluginBase->execute() (Line: 196)
Drupal\views\Plugin\views\display\Page->execute() (Line: 1635)
Drupal\views\ViewExecutable->executeDisplay('page_1', Array) (Line: 81)
Drupal\views\Element\View::preRenderViewElement(Array)
call_user_func_array(Array, Array) (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback(Array, Array, 'Render #pre_render callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was %s. See
https://www.drupal.org/node/2966725 →
', 'exception', 'Drupal\Core\Render\Element\RenderCallbackInterface') (Line: 788)
Drupal\Core\Render\Renderer->doCallback('#pre_render', Array, Array) (Line: 374)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 204)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 242)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 243)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 132)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 174)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
Reverting to 4.0.0-alpha1 the problem goes away.