BookManager->addFormElements needs to check for book before adding elements

Problem/Motivation

With book, media, and media library installed, the "Add media" form sometimes fails.

Steps to Reproduce

1. Drupal 8.7.1, PHP 7.1.12
2. Enable book, media, and media_library.
3. Add a reference field of type Media, allowing images, on a basic page. Use the Media library widget on the form display.
4. Add a basic page and upload or select an image from the media library.
5. Edit that page and remove the image, then try to upload or select a different image.
6. The image thumbnail fails to appear, and an error from BookManager appears in the console log and in recent log messages.

Note that the content does not need to be in a book in order for this failure to occur.

Location: /node/1/edit?_wrapper_format=drupal_ajax&ajax_form=1

Message:

TypeError: Argument 1 passed to Drupal\book\BookManager::addParentSelectFormElements() must be of the type array, null given, called in /drupal-8.7.1/core/modules/book/src/BookManager.php on line 201 in Drupal\book\BookManager->addParentSelectFormElements() (line 357 of /drupal-8.7.1/core/modules/book/src/BookManager.php) #0 /drupal-8.7.1/core/modules/book/src/BookManager.php(201): Drupal\book\BookManager->addParentSelectFormElements(NULL) #1 /drupal-8.7.1/core/modules/book/book.module(150): Drupal\book\BookManager->addFormElements(Array, Object(Drupal\Core\Form\FormState), Object(Drupal\node\Entity\Node), Object(Drupal\Core\Session\AccountProxy), true) #2 /drupal-8.7.1/core/lib/Drupal/Core/Extension/ModuleHandler.php(539): book_form_node_form_alter(Array, Object(Drupal\Core\Form\FormState), 'node_page_edit_...') #3 /drupal-8.7.1/core/lib/Drupal/Core/Form/FormBuilder.php(835): Drupal\Core\Extension\ModuleHandler->alter('form', Array, Object(Drupal\Core\Form\FormState), 'node_page_edit_...') #4 /drupal-8.7.1/core/lib/Drupal/Core/Form/FormBuilder.php(417): Drupal\Core\Form\FormBuilder->prepareForm('node_page_edit_...', Array, Object(Drupal\Core\Form\FormState)) #5 /drupal-8.7.1/core/lib/Drupal/Core/Form/FormBuilder.php(626): Drupal\Core\Form\FormBuilder->rebuildForm('node_page_edit_...', Object(Drupal\Core\Form\FormState), Array) #6 /drupal-8.7.1/core/lib/Drupal/Core/Form/FormBuilder.php(319): Drupal\Core\Form\FormBuilder->processForm('node_page_edit_...', Array, Object(Drupal\Core\Form\FormState)) #7 /drupal-8.7.1/core/lib/Drupal/Core/Controller/FormController.php(93): Drupal\Core\Form\FormBuilder->buildForm('node_page_edit_...', Object(Drupal\Core\Form\FormState)) #8 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch)) #9 /drupal-8.7.1/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array) #10 /drupal-8.7.1/core/lib/Drupal/Core/Render/Renderer.php(582): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #11 /drupal-8.7.1/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure)) #12 /drupal-8.7.1/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) #13 /drupal-8.7.1/vendor/symfony/http-kernel/HttpKernel.php(151): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #14 /drupal-8.7.1/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #15 /drupal-8.7.1/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #16 /drupal-8.7.1/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #17 /drupal-8.7.1/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #18 /drupal-8.7.1/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #19 /drupal-8.7.1/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #20 /drupal-8.7.1/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #21 /drupal-8.7.1/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #22 /drupal-8.7.1/core/lib/Drupal/Core/DrupalKernel.php(693): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #23 /drupal-8.7.1/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #24 {main}.

Proposed resolution

An if ($node->book) statement around the following pieces of BookManager->addFormElements seems to make everything work properly again:

$form['book'] = [
  '#type' => 'details',
  '#title' => $this->t('Book outline'),
  '#weight' => 10,
  '#open' => !$collapsed,
  '#group' => 'advanced',
  '#attributes' => [
    'class' => ['book-outline-form'],
  ],
  '#attached' => [
    'library' => ['book/drupal.book'],
  ],
  '#tree' => TRUE,
];
foreach (['nid', 'has_children', 'original_bid', 'parent_depth_limit'] as $key) {
  $form['book'][$key] = [
    '#type' => 'value',
    '#value' => $node->book[$key],
  ];
}

$form['book']['pid'] = $this->addParentSelectFormElements($node->book);