Thanks, that worked! However, I did run into some other problems. When I tried to delete a message in editor, it came up with a similar error message and crashed. (Of course, the solution is to edit the messages in the editor, rather than delete them, but thought I'd mention it.)
Secondly, and it's not really a problem with this plugin, but I'm getting a really annoying conflict with Yanfly's Message Core. Basically, with that plugin enabled only half of the message appears. I thought it might have been limited by the number of message rows, but using the message row plugin command from the Yanfly plugin to expand it didn't work. *sigh* It's not like I built every cutscene up to know with the Message Core functionality, not at all...