TrustedCallbackInterface

Problem/Motivation

SubscribeForm class must implement TrustedCallbackInterface for D9 compatibility.

Notice: Undefined variable: value in Drupal\node_subscription\Form\SubscribeForm->validateForm() (line 91 of modules/contrib/node_subscription/src/Form/SubscribeForm.php).
Drupal\node_subscription\Form\SubscribeForm->validateForm(Array, Object)
call_user_func_array(Array, Array) (Line: 82)
Drupal\Core\Form\FormValidator->executeValidateHandlers(Array, Object) (Line: 273)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'subscribe_form') (Line: 118)
Drupal\Core\Form\FormValidator->validateForm('subscribe_form', Array, Object) (Line: 589)
Drupal\Core\Form\FormBuilder->processForm('subscribe_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm('Drupal\node_subscription\Form\SubscribeForm', Object) (Line: 219)
Drupal\Core\Form\FormBuilder->getForm('Drupal\node_subscription\Form\SubscribeForm') (Line: 31)
Drupal\node_subscription\Plugin\Block\SubscribeBlock->build() (Line: 171)
Drupal\block\BlockViewBuilder::preRender(Array)
call_user_func_array(Array, Array) (Line: 100)
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. Support for this callback implementation is deprecated in 8.8.0 and will be removed in Drupal 9.0.0. See https://www.drupal.org/node/2966725', 'silenced_deprecation', 'Drupal\Core\Render\Element\RenderCallbackInterface') (Line: 781)
Drupal\Core\Render\Renderer->doCallback('#pre_render', Array, Array) (Line: 372)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 444)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 200)
Drupal\Core\Render\Renderer->render(Array) (Line: 501)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 93)
__TwigTemplate_ef4b334d5a2ad31fbb2b1f1a956d185491d7dd180401d13a8faa1292e966487f->doDisplay(Array, Array) (Line: 455)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 422)
Twig\Template->display(Array) (Line: 434)
Twig\Template->render(Array) (Line: 64)
twig_render_template('core/themes/seven/templates/page.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('page', Array) (Line: 431)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 200)
Drupal\Core\Render\Renderer->render(Array) (Line: 501)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 102)
__TwigTemplate_d0dd67d1828842eae2b5a31ae713a5fd7f0c06568de21df9cd467bf9a8ec8088->doDisplay(Array, Array) (Line: 455)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 422)
Twig\Template->display(Array) (Line: 434)
Twig\Template->render(Array) (Line: 64)
twig_render_template('core/themes/seven/templates/classy/layout/html.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 431)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 200)
Drupal\Core\Render\Renderer->render(Array) (Line: 147)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
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: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 156)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
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: 130)
Drupal\cdn\StackMiddleware\DuplicateContentPreventionMiddleware->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Save form with subscribe button without selecting any options.

Proposed resolution

Add implementation for SubscribeForm and add method as follows:

  public static function trustedCallbacks(): array {
    return ['buildForm','validateForm','validateCallback'];
  }