Warning: Undefined array key "#webform_key" in WebformManagedFileBase.php

Same error here.

Related issue with a similar approximation https://www.drupal.org/project/webform/issues/3343209

Full stack:

Message	Warning: Undefined array key "#webform_key" in Drupal\webform\Plugin\WebformElement\WebformManagedFileBase->prepare() (line 301 of /mysite/docroot/modules/contrib/webform/src/Plugin/WebformElement/WebformManagedFileBase.php) #0 /mysite/docroot/core/includes/bootstrap.inc(166): _drupal_error_handler_real() #1 /mysite/docroot/modules/contrib/webform/src/Plugin/WebformElement/WebformManagedFileBase.php(301): _drupal_error_handler() #2 /mysite/docroot/modules/contrib/webform/modules/webform_ui/src/Form/WebformUiElementTypeFormBase.php(308): Drupal\webform\Plugin\WebformElement\WebformManagedFileBase->prepare() #3 /mysite/docroot/modules/contrib/webform/modules/webform_ui/src/Form/WebformUiElementTypeFormBase.php(246): Drupal\webform_ui\Form\WebformUiElementTypeFormBase->buildElementPreview() #4 /mysite/docroot/modules/contrib/webform/modules/webform_ui/src/Form/WebformUiElementTypeSelectForm.php(78): Drupal\webform_ui\Form\WebformUiElementTypeFormBase->buildRow() #5 [internal function]: Drupal\webform_ui\Form\WebformUiElementTypeSelectForm->buildForm() #6 /mysite/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(536): call_user_func_array() #7 /mysite/docroot/core/lib/Drupal/Core/Form/FormBuilder.php(284): Drupal\Core\Form\FormBuilder->retrieveForm() #8 /mysite/docroot/core/lib/Drupal/Core/Controller/FormController.php(73): Drupal\Core\Form\FormBuilder->buildForm() #9 [internal function]: Drupal\Core\Controller\FormController->getContentResult() #10 /mysite/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array() #11 /mysite/docroot/core/lib/Drupal/Core/Render/Renderer.php(669): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\@closure() #12 /mysite/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext() #13 /mysite/docroot/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() #14 /mysite/vendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\@closure() #15 /mysite/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw() #16 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpKernel\HttpKernel->handle() #17 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle() #18 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle() #19 /mysite/vendor/asm89/stack-cors/src/Cors.php(53): Drupal\Core\StackMiddleware\ContentLength->handle() #20 /mysite/docroot/modules/contrib/shield/src/ShieldMiddleware.php(263): Asm89\Stack\Cors->handle() #21 /mysite/docroot/modules/contrib/shield/src/ShieldMiddleware.php(130): Drupal\shield\ShieldMiddleware->bypass() #22 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\shield\ShieldMiddleware->handle() #23 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() #24 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() #25 /mysite/docroot/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle() #26 /mysite/docroot/core/lib/Drupal/Core/DrupalKernel.php(741): Drupal\Core\StackMiddleware\StackedHttpKernel->handle() #27 /mysite/docroot/index.php(19): Drupal\Core\DrupalKernel->handle() #28 @main.

Thanks for the patch.

Please put the patch into an issue fork and merge request.

Also running into this issue and the patch resolved the problem for me. Running: Webform 6.2.9 on Drupal 10.4.1

Created MR

Patch seems very safe.

Thanks for the patch! It successfully prevents the warnings.
However, does anyone have an explanation for why the #webform_key is missing? It seems like it has not been initialized properly or something, but I couldn't find the root cause. (Since it doesn’t break any functionality, I haven’t investigated further.)