Assertion error on multi-step form node creation

ERROR: AssertionError: Cannot load a NULL ID. in assert() (line 249 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).

Where I encountered it / rough steps to reproduce:
The Drupal 8 group module has a two step form for creating content in a group. The first step (page) is to create the node. The second step creates the group content relationships (separate entity).

The node is not actually created until the entire process is complete.

Therefore, on initial node form submit, there is no NID, as the node has not been saved. The error is as follows:

 AssertionError: Cannot load a NULL ID. in assert() (line 249 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
assert(, 'Cannot load a NULL ID.') (Line: 249)
Drupal\Core\Entity\EntityStorageBase->load(NULL) (Line: 645)
Drupal\simple_sitemap\Simplesitemap->setEntityInstanceSettings('node', NULL, Array) (Line: 128)
simple_sitemap_entity_form_submit(Array, Object)
call_user_func_array('simple_sitemap_entity_form_submit', Array) (Line: 111)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 590)
Drupal\Core\Form\FormBuilder->processForm('node_basic_page_form', Array, Object) (Line: 319)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 61)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object, 'default', Array) (Line: 361)
Drupal\group\Entity\Controller\GroupContentController->createForm(Object, 'group_node:basic_page')
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
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: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->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: 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: 693)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)