Thank you for the report. Any chance that you may have attempted the battle again? Do the same bug still occur?
I've identified the issue. It turns out that if the party formation has been changed, (despite Begonia being the only party member), this error will occur. For the time being, if you start a new file without adjusting party formation at all until that point, you would be fine. Thank you again for the report, it was very helpful for me to diagnose the error.