Hey there.
This appears to be an issue with a change I implemented a while back. Originally I had modified the actor.isDualWield() function to return false in battle if the actor was dual wield type, but wasn't currently wielding two weapons. A couple of updates ago, I undid this change, and replaced most of those checks with a new function actor.isCurrentlyDualWielding(), the purpose of this change was so that in battle equipment changing plugins didn't incorrectly determine that an actor couldn't wield an offhand weapon.
It appears after I made this change, the function causing your crash was not updated. With the original change to the dual wield function, it would not have checked the wtype of the offhand item if the actor was barehanded or using a shield, but now it does both.
I have uploaded a fix for this, in the newest version. The second issue you posted about below is also because of the same thing, the function that determines if an actor is dual wielding, and adds the second attack action was also checking if they were dual wield type, and not if they were currently dual wielding.
~Ramza