We've hit this recently on our demo installation, but I believe we've stumbled upon this in the past in different parts of Koha that may interact with Items (and Bookings, by extension).
The error log message is the following: GET /api/v1/preservation/waiting-list/items: unhandled exception (Mojo::Exception)<<Can't call method "bookable" on an undefined value at /usr/share/koha/lib/Koha/Item.pm line 1929.>> Produced when accessing: <staff_url>/cgi-bin/koha/preservation/waiting-list With one or more (?) items without an itemtype (or itype, depending on preference?) More analysis required.
Created attachment 184897 [details] [review] Bug 40555: Consider possibility of item not having itemtype on effective_bookable
IMO This is invalid. We have discussed on bug 33166 (and others) and decided that we should not deal with corrupted data. The bug here is to have the column null at the DB level.