Hey there.
The unshift crash happens if you're not using BattleCore. In the very early versions of the plugin, including the one in the demo folder, the BattleCore wasn't technically required, although the plugin still visually looked a lot better when BattleCore was used. In an early update, one of the changes I made to the plugin to make it more plug and play friendly adds a bunch of action sequence commands to the attack skill (and other dual wield skills) which reset some counters and perform some extra clean up after a dual wield attack.
For the most part, this is invisible to the end user, but if you're not using BattleCore, it causes this error.
~Ramza