Fatal error when creating new node with IPE enabled

Problem/Motivation

Testing with Layout Builder IPE 1.0.x-dev on a clean instance of Drupal 9.4.8.

When I attempt to create a new node for a content type with Layout Builder IPE enabled, I get a fatal error:

The website encountered an unexpected error. Please try again later.
AssertionError: Cannot load the "node" entity with NULL ID. in assert() (line 295 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
assert(, 'Cannot load the "node" entity with NULL ID.') (Line: 295)
Drupal\Core\Entity\EntityStorageBase->load(NULL) (Line: 1151)
Drupal\Core\Entity\ContentEntityStorageBase->loadUnchanged(NULL) (Line: 175)
layout_builder_ipe_form_alter(Array, Object, 'node_page_form') (Line: 562)
Drupal\Core\Extension\ModuleHandler->alter('form', Array, Object, 'node_page_form') (Line: 835)
Drupal\Core\Form\FormBuilder->prepareForm('node_page_form', Array, Object) (Line: 279)
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: 564)
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: 159)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 709)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

1. Install Layout Builder IPE
2. Visit Administration > Structure> Content types > Basic page > Manage display > Full content
3. Check the Use Layout Builder, Allow each content item to have its layout customized, and Use the Layout Builder IPE frontend options, and Save settings
4. Attempt to add a new Basic Page node

Proposed resolution

Line 171 of layout_builder_ipe_form_alter() should check if $entity-->isNew() before using $entity-->id() (which will be null for new entities).

Remaining tasks

1. Patch
2. Review
3. Commit

User interface changes

None

API changes

None

Data model changes

None