Warning: Trying to access array offset on value of type null in Drupal\book\BookManager->bookSubtreeData() (line 1147 of core/modules/book/src/BookManager.php).
Warning: Trying to access array offset on value of type null in Drupal\book\BookOutlineStorage->getBookSubtree() (line 190 of core/modules/book/src/BookOutlineStorage.php).
Warning: Trying to access array offset on value of type null in Drupal\book\BookOutlineStorage->getBookSubtree() (line 192 of core/modules/book/src/BookOutlineStorage.php).
Warning: Trying to access array offset on value of type null in Drupal\book\BookManager->bookSubtreeData() (line 1159 of core/modules/book/src/BookManager.php).
Warning: Trying to access array offset on value of type null in Drupal\book\BookManager->bookSubtreeData() (line 1166 of core/modules/book/src/BookManager.php).
Warning: Trying to access array offset on value of type null in Drupal\book\Form\BookAdminEditForm->bookAdminTable() (line 187 of core/modules/book/src/Form/BookAdminEditForm.php).
