Just updated the mod from mod version 1.3.0. to 1.4.0, I had used an existing remote to call a spidertron to a destination, waypoint mode and patrol mode both off. I then cleared cursor with q and picked up a BP book. As soon as I started to shift scroll through the book, the non-recoverable error occurred.

Error:
The mod Spidertron Waypoints (1.4.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SpidertronWaypoints::waypoints-mode-scroll-backwards (ID 185)
LuaItemStack API call when LuaItemStack was invalid for read.
stack traceback:
SpidertronWaypoints/mode_handling.lua:134: in function 'on_mode_scrolled'
SpidertronWaypoints/mode_handling.lua:149: in function <SpidertronWaypoints/mode_handling.lua:149>
stack traceback:
[C]: in function 'index'
__SpidertronWaypoints/mode_handling.lua:134: in function 'on_mode_scrolled'
SpidertronWaypoints/mode_handling.lua:149: in function <SpidertronWaypoints/mode_handling.lua:149>

Just did a little more testing, and the error occurs even without setting a waypoint and while just trying to scroll through any book, even on saves where spidertrons aren't even researched

Just did a little more testing, and the error occurs whenever scrolling through a blueprint book that is in the BP library, even on saves without spidertrons or remotes. doesn't error with BP books in the inventory.

Thanks for the report. That looks like a simple oversight on my part. Should be a simple fix :)

Fixed in 1.4.1, sorry for any inconvenience!