Support InlineBlock block plugin

Problem/Motivation

As a site builder, I want to allow the creation of blocks on an entity, where these blocks will be single use. This is similar to adding blocks in layout_builder and the block type involved is InlineBlock. This issue is not about using layout builder though, it happens that layout builder provides the InlineBlock plugin.

I've added a general question about my problem on stack exchange as well.

Steps to reproduce

1. Install block_field, layout_builder
2. Create a custom block type with one field, just some text
3. (Do not create any blocks in the custom block library)
4. New node type "page"
5. Add a block_field field to "page", and reference the new plugin, which will be type "inline block" (provider is Layout Builder)
6. Create a node, populate by choose the block type and filling in the form.
7. Hit save

Resulting error :

The website encountered an unexpected error. Please try again later.
TypeError: Argument 1 passed to Drupal\Component\Utility\NestedArray::getValue() must be of the type array, null given, called in /app/web/core/lib/Drupal/Core/Field/WidgetBase.php on line 414 in Drupal\Component\Utility\NestedArray::getValue() (line 69 of core/lib/Drupal/Component/Utility/NestedArray.php).
Drupal\Component\Utility\NestedArray::getValue(NULL, NULL) (Line: 414)
Drupal\Core\Field\WidgetBase->flagErrors(Object, Object, Array, Object) (Line: 265)
Drupal\Core\Entity\Entity\EntityFormDisplay->flagWidgetsErrorsFromViolations(Object, Array, Object) (Line: 252)
Drupal\Core\Entity\Entity\EntityFormDisplay->validateFormValues(Object, Array, Object) (Line: 181)
Drupal\layout_builder\Plugin\Block\InlineBlock->blockValidate(Array, Object) (Line: 207)
Drupal\Core\Block\BlockBase->validateConfigurationForm(Array, Object) (Line: 238)
Drupal\block_field\Plugin\Field\FieldWidget\BlockFieldWidget::validate(Array, Object, Array)
call_user_func_array(Array, Array) (Line: 280)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 236)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 236)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 236)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'node_article_form') (Line: 118)
Drupal\Core\Form\FormValidator->validateForm('node_article_form', Array, Object) (Line: 589)
Drupal\Core\Form\FormBuilder->processForm('node_article_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object) (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 67)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->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: 49)
Asm89\Stack\Cors->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: 705)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

It would be great if this module supported the core "InlineBlock" block plugin type.

Remaining tasks

User interface changes

API changes

Data model changes