Greetings again. I have corrected the problem.
I had thought that I had tested the logic quite thoroughly on my own, but it turns out I must've skipped the checks to see if having no weapons in either hand would cause a problem. I have added the required checks, and the project should no longer crash when either of the actor's hands are empty, when using a skill that requires a specific weapon type. Please download the updated plugin file at your earliest convenience.