Hello there.
It would appear that this is an uncaught bug caused in v2.55, when I added some checks to lock main or offhand weapons on actors who were using skills that required a specific weapon type. It looks like after this update I was also checking for locked weapon types on enemies, who don't use weapons, and therefore throw this error.
I have uploaded a fix, and apologize for the inconvenience. The reason why it worked in my demo project was because the uploaded demo project is using an older version of the plugin. Redownloading the plugin file from the download page will show it updated to v2.58 with this fix included.
Thanks for reporting this issue.