Maybe it's due to different character setup where not all solvents are known or not all alchemy levels etc. were unlocked already.
You could add a ~= nil check to prevent the error message and instead send a message with the information you need to the chat and let users send this message to you.
Or use LibDebugLogger in your addon to log those errors to the SavedVariables and let them send you teh file or uplaod them at sirinsidiators LibDebugLogger online viewer:
https://sir.insidi.at/or/logviewer/