function interact(event) {
    var role = event.npc.role;
    var World = event.npc.world;
    var items = []
    items[0] = World.createItem('minecraft:stone',0,1);
    items[1] = World.createItem('minecraft:air',0,1);
    items[2] = World.createItem('minecraft:log',0,1);
    /*both result in a same error*/
    role.set(0,items[0],items[1],items[2]);
    role.set(1,items[0],null,items[2]);
    /*this works fine*/
    role.set(1,items[0],items[0],items[2]);
}