Adding a target results in: 'TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "Drupal\dfp\Form\Tag"'

Problem/Motivation

TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "Drupal\dfp\Form\Tag" not found or invalid function name in call_user_func_array() (Line 129 in ...../web/core/lib/Drupal/Core/Form/FormSubmitter.php).

Backtrace:

#0 ....../web/core/lib/Drupal/Core/Form/FormSubmitter.php(129): call_user_func_array()
#1 ....../web/core/lib/Drupal/Core/Form/FormSubmitter.php(67): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers()
#2 ....../web/core/lib/Drupal/Core/Form/FormBuilder.php(597): Drupal\Core\Form\FormSubmitter->doSubmitForm()
#3 ....../web/core/lib/Drupal/Core/Form/FormBuilder.php(325): Drupal\Core\Form\FormBuilder->processForm()
#4 ....../web/core/lib/Drupal/Core/Controller/FormController.php(73): Drupal\Core\Form\FormBuilder->buildForm()
#5 ....../web/core/modules/layout_builder/src/Controller/LayoutBuilderHtmlEntityFormController.php(39): Drupal\Core\Controller\FormController->getContentResult()
#6 [internal function]: Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult()
#7 ....../web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array()
#8 ....../web/core/lib/Drupal/Core/Render/Renderer.php(627): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#9 ....../web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext()
#10 ....../web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext()
#11 ....../vendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#12 ....../vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw()
#13 ....../web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\Component\HttpKernel\HttpKernel->handle()
#14 ....../web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle()
#15 ....../web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle()
#16 ....../web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass()
#17 ....../web/core/modules/ban/src/BanMiddleware.php(50): Drupal\page_cache\StackMiddleware\PageCache->handle()
#18 ....../web/modules/contrib/shield/src/ShieldMiddleware.php(270): Drupal\ban\BanMiddleware->handle()
#19 ....../web/modules/contrib/shield/src/ShieldMiddleware.php(169): Drupal\shield\ShieldMiddleware->bypass()
#20 ....../web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\shield\ShieldMiddleware->handle()
#21 ....../web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#22 ....../web/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#23 ....../web/modules/contrib/remove_http_headers/src/StackMiddleware/RemoveHttpHeadersMiddleware.php(49): Drupal\Core\StackMiddleware\AjaxPageState->handle()
#24 ....../web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\remove_http_headers\StackMiddleware\RemoveHttpHeadersMiddleware->handle()
#25 ....../web/core/lib/Drupal/Core/DrupalKernel.php(704): Drupal\Core\StackMiddleware\StackedHttpKernel->handle()
#26 ....../web/index.php(19): Drupal\Core\DrupalKernel->handle()
#27 {main}

Steps to reproduce

1. Add new DFP Tag
2. Fill the fields, used data:
   • Slot Name: xxx_medium_rectangle_300x250_ba1
   • Sizes: 300x250
   • Unit Pattern: xxx_medium_rectangle_300x250_ba1
   • Title: Advert
   • Create a block for this tag: true
   • Targeting: xxx_position: 1
3. Click "add another target" => See ajax and log error

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes