User warning:
Missing proxy class 'Drupal\book_tree_menu\ProxyClass\oscBookManager' for lazy service 'book.manager'.
Use the following command to generate the proxy class:
php core/scripts/generate-proxy-class.php 'Drupal\book_tree_menu\oscBookManager' "modules/contrib/book_tree_menu/src"
in Drupal\Core\DependencyInjection\Compiler\ProxyServicesPass->process() (line 66 of core/lib/Drupal/Core/DependencyInjection/Compiler/ProxyServicesPass.php).