Same problem, you cannot use online blocks because when saving the following error appears:
The website encountered an unexpected error. Try again later.
TypeError: Drupal\Component\Utility\NestedArray::getValue(): Argument #1 ($array) must be of type array, null given, called in /var/www/example.com/core/lib/Drupal/Core/Field/WidgetBase.php on line 531 in Drupal\Component\Utility\NestedArray::getValue() (line 69 of core/lib/Drupal/Component/Utility/NestedArray.php).
Drupal\Core\Field\WidgetBase->flagErrors() (Line: 277)
Drupal\Core\Entity\Entity\EntityFormDisplay->flagWidgetsErrorsFromViolations() (Line: 264)
Drupal\Core\Entity\Entity\EntityFormDisplay->validateFormValues() (Line: 182)
Drupal\layout_builder\Plugin\Block\InlineBlock->blockValidate() (Line: 211)
Drupal\Core\Block\BlockBase->validateConfigurationForm() (Line: 249)
Drupal\block_field\Plugin\Field\FieldWidget\BlockFieldWidget::validate()
call_user_func_array() (Line: 281)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 118)
Drupal\Core\Form\FormValidator->validateForm() (Line: 593)
Drupal\Core\Form\FormBuilder->processForm() (Line: 326)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult() (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)The error that I explained in comment #8 led me here 🐛 Argument #1 ($values) must be of type array, null given. Postponed: needs info to apply the patch and if there is any problem with the layout builder