Path validation constraint prevents saving of node that uses entitygroupfield

This patch is causing an issue when I create a node and add to a group using /node/add route:

The website encountered an unexpected error. Please try again later.
TypeError: Drupal\Component\Utility\NestedArray::getValue(): Argument #2 ($parents) must be of type array, null given, called in /app/docroot/core/lib/Drupal/Core/Field/WidgetBase.php on line 426 in Drupal\Component\Utility\NestedArray::getValue() (line 69 of core/lib/Drupal/Component/Utility/NestedArray.php).
Drupal\Component\Utility\NestedArray::getValue(Array, NULL) (Line: 426)
Drupal\Core\Field\WidgetBase->flagErrors(Object, Object, Array, Object) (Line: 268)
Drupal\Core\Entity\Entity\EntityFormDisplay->flagWidgetsErrorsFromViolations(Object, Array, Object) (Line: 255)
Drupal\Core\Entity\Entity\EntityFormDisplay->validateFormValues(Object, Array, Object) (Line: 762)
Drupal\entitygroupfield\Plugin\Field\FieldWidget\EntityGroupFieldWidgetBase->elementValidate(Array, Object, Array)
call_user_func_array(Array, Array) (Line: 282)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'node_page_form') (Line: 118)
Drupal\Core\Form\FormValidator->validateForm('node_page_form', Array, Object) (Line: 588)
Drupal\Core\Form\FormBuilder->processForm('node_page_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->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: 50)
Drupal\ban\BanMiddleware->handle(Object, 1, 1) (Line: 265)
Drupal\shield\ShieldMiddleware->bypass(Object, 1, 1) (Line: 132)
Drupal\shield\ShieldMiddleware->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)

I thought it was permission related but I gave the editor role full permissions in both global and group permissions and that error would still happen. I have no issues when I create a node using the /group/4/content/create/group_node route. In this case, my editor role is a mapped to the global editor role.

It's been about 2 years since I looked at any of this code 😅, but doesn't the widget have the notion of sort of a read-only display for folks who can see the group associations but not edit them? Is that what's causing the trouble here? Can/should we implement that in a different way so that we're not actually directly using the whole FormAPI to add read-only markup?

Regardless, it'd be great to have some automated test coverage to demonstrate the bug, both so we know we're fixing it, and so we're sure we don't break it in the future.

Thanks,
-Derek