ke_search indexing exception with cart_books 4.0.4

[vmans]

Not a complete fix yet, though it does not throw a error:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: strlen(): Argument #1 ($string) must be of type string, int given | TypeError thrown in file /var/www/clients/client2/web19/web/typo3conf/ext/cart_books/Classes/Hooks/KeSearchIndexer.php in line 130. Requested URL: https://etuconsult.nl/typo3/module/web/KeSearchBackendModule?token=--AnonymizedToken--&id=11&do=startindexer

11LTS
ke_search 4.5.1
cart_books 4.0.4