When facing an issue like this, the first step is to check whether any getText methods have been used instead of getValue.
Next, verify if the value is sourced from another field. If it is, you should retrieve the value using a search.
I encountered this error while executing my Map/Reduce script. Although the error appeared in my script, it was actually caused by another user event script on the same record (beforeSubmit). After reviewing that script, I resolved the issue by replacing getText with getValue.