Error message

Warning: Undefined array key "title" in template_preprocess_book_navigation() (line 492 of modules/contrib/book/book.module).
template_preprocess_book_navigation()
call_user_func_array() (Line: 261)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100)
__TwigTemplate_5007bcbc790ebcb7a8e2f3b55cc84482->block_content() (Line: 432)
Twig\Template->yieldBlock() (Line: 79)
__TwigTemplate_5007bcbc790ebcb7a8e2f3b55cc84482->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 503)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 57)
__TwigTemplate_91258dbe6da00a1367258ab21df47d58->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 503)
Drupal\Core\Render\Renderer->doRender() (Line: 503)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100)
__TwigTemplate_cd640601ed95304c7cf1a019616696c9->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 55)
__TwigTemplate_511bb94268366eeff118f12397387cdd->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 503)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 86)
__TwigTemplate_c68cc841a714633b3405d51364d23f6e->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 490)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 153)
Drupal\Core\Render\Renderer->Drupal\Core\Render\{closure}() (Line: 637)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 154)
Drupal\Core\Render\Renderer->renderRoot() (Line: 139)
Drupal\book\Controller\BookController->bookExport()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 637)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 201)
Drupal\page_cache\StackMiddleware\PageCache->fetch() (Line: 138)
Drupal\page_cache\StackMiddleware\PageCache->lookup() (Line: 87)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 153)
Drupal\cryptolog\CryptologMiddleware->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)
Kristian Falk

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