This happened when I tried to put a blueprint book with train stuff into my Personal Book, then saved and loaded the game.
https://pastebin.com/NRHiAsYg

12569.164 Script @blueprint-sandboxes/scripts/isolation.lua:22: Detected isolation level: none
12583.411 Script @sbPersonalBook/control.lua:74: safecall error:
LuaItemStack API call when LuaItemStack was invalid for read.
stack traceback:
sbPersonalBook/control.lua:45: in function <sbPersonalBook/control.lua:41>
[C]: in function 'index'
__sbPersonalBook/control.lua:463: in function 'walk'
sbPersonalBook/control.lua:473: in function 'pb_items_in_book'
sbPersonalBook/control.lua:777: in function <sbPersonalBook/control.lua:768>
[C]: in function 'xpcall'
sbPersonalBook/control.lua:72: in function 'safecall'
sbPersonalBook/control.lua:768: in function <sbPersonalBook/control.lua:763>
(...tail calls...)
[C]: in function 'xpcall'
sbPersonalBook/control.lua:72: in function <sbPersonalBook/control.lua:71>
(...tail calls...)
12764.643 Info AppManager.cpp:323: Saving to _autosave2 (blocking).
12766.863 Info AppManagerStates.cpp:2110: Saving finished

thanks,
since i use my sorting mod almost always, some instances where books could contain empty inventory slots, slipped by me.

i hope that was the last spot which needed fixing.

I got another error today:
16486.168 Script @blueprint-sandboxes/scripts/isolation.lua:22: Detected isolation level: none
16487.215 Script @sbPersonalBook/control.lua:73: safecall error:
'active_index': real number expected got nil.
stack traceback:
sbPersonalBook/control.lua:44: in function <sbPersonalBook/control.lua:40>
[C]: in function 'newindex'
__sbPersonalBook/control.lua:872: in function 'func'
sbPersonalBook/control.lua:541: in function 'pb_recurse_func'
sbPersonalBook/control.lua:539: in function 'pb_recurse_func'
sbPersonalBook/control.lua:539: in function 'pb_recurse_func'
sbPersonalBook/control.lua:870: in function <sbPersonalBook/control.lua:812>
[C]: in function 'xpcall'
sbPersonalBook/control.lua:71: in function 'safecall'
sbPersonalBook/control.lua:812: in function <sbPersonalBook/control.lua:807>
(...tail calls...)
[C]: in function 'xpcall'
sbPersonalBook/control.lua:71: in function <sbPersonalBook/control.lua:70>
(...tail calls...)

thanks,
forgot that active_index can be invalid for empty books.

added sanity checks, should work now.