refactor: use an explicit null check

Because we never want the table inside the WeakTableRef to be possible to be null in this case, so it's better to throw an error in that case.

msu/hooks/items/item_container.nut

@@ -40,7 +40,7 @@
 
 		local actionCost = this.getActionCost(_items);
 		this.m.Actor.setActionPoints(::Math.max(0, this.m.Actor.getActionPoints() - actionCost));
-		this.m.Actor.getSkills().onPayForItemAction(::MSU.isNull(this.m.ActionSkill) ? null : this.m.ActionSkill.get(), _items);
+		this.m.Actor.getSkills().onPayForItemAction(this.m.ActionSkill == null ? null : this.m.ActionSkill.get(), _items);
 		this.m.ActionSkill = null;
 	}