Notice: Undefined index: #parents when placed as a paragraph.

Problem/Motivation

When the the map field widget is added to a paragraph entity as well as the address field, the widget is still looking for the address in the global form that the paragraph is embedded in, resulting in missing address field, as well as the following notice messages during submission.

Notice: Undefined index: #parents in Drupal\Core\Form\FormState->getError() (line 1112 of core/lib/Drupal/Core/Form/FormState.php).
Drupal\Core\Form\FormState->getError(Array) (Line: 160)
Drupal\Core\Form\FormErrorHandler->setElementErrorsFromFormState(Array, Object, Array) (Line: 120)
Drupal\Core\Form\FormErrorHandler->setElementErrorsFromFormState(Array, Object, Array) (Line: 120)
Drupal\Core\Form\FormErrorHandler->setElementErrorsFromFormState(Array, Object, Array) (Line: 120)
Drupal\Core\Form\FormErrorHandler->setElementErrorsFromFormState(Array, Object, Array) (Line: 120)
Drupal\Core\Form\FormErrorHandler->setElementErrorsFromFormState(Array, Object) (Line: 23)
Drupal\Core\Form\FormErrorHandler->handleFormErrors(Array, Object) (Line: 202)
Drupal\Core\Form\FormValidator->finalizeValidation(Array, Object, 'node_membership_form') (Line: 120)
Drupal\Core\Form\FormValidator->validateForm('node_membership_form', Array, Object) (Line: 571)
Drupal\Core\Form\FormBuilder->processForm('node_membership_form', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm('node_membership_form', Object) (Line: 48)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object) (Line: 113)
Drupal\node\Controller\NodeController->add(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 576)
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}()
call_user_func_array(Object, Array) (Line: 153)
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: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 657)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

Have the widget search for address field in the same entity form element.
For example:
1. If both are fields on the entity - work directly there. (curently works)
2. If both are fields on a sub entity (paragraph) - look in the sub-form for that entity for the address field. (currently breaks)

Open questions are still how to handle cross entity selection.
An option is to change the widget settings to point to the full name of the address element, so it can be anywhere in the form. This will have a complex upgrade path...

This can be achieved through the #array_parents and some management there.

Remaining tasks

Patch - coming shortly. I have a working solution locally.
Reviews...
Commit...

User interface changes

None.

API changes

None.

Data model changes

None.