Errors, when adding new book child

When I add a child page to a book page that does not yet have childs, an error occurs:

Notice: Undefined variable: lastWeight in Drupal\book_link_weight\Form\BookLinkWeightForm->alterBookOutlineForm() (line 85 of modules/book_link_weight/src/Form/BookLinkWeightForm.php).

Drupal\book_link_weight\Form\BookLinkWeightForm->alterBookOutlineForm(Array, Object) (Line: 15)
book_link_weight_form_node_form_alter(Array, Object, 'node_book_form') (Line: 539)
Drupal\Core\Extension\ModuleHandler->alter('form', Array, Object, 'node_book_form') (Line: 836)
Drupal\Core\Form\FormBuilder->prepareForm('node_book_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: 573)
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: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
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: 706)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

If parent page already have any childs error does not occurs.

Sorry for my english )

And I'm sorry - I don't know how to properly ask for help. You write that you were inspired by the Menu Link Weight module to create this module. Menu Link Weight module has an integration with the Client-side Hierarchical Select module, which simplifies the selection of the parent menu item - is it possible to do the same integration in your module in order to simplify the selection of the parent book page?