question

The header of the trigger option gets a reference to the conveyor item representation of the triggering item. If the item was already destroyed when the callback to release/exit the area happens the invalid reference error happens.

Releasing the area does not require this reference, so if you do not intend to use any other pick options in this trigger, then you can simply comment out the fourth line of the header. Otherwise you can adjust it to the following to prevent the error from popping up.

Conveyor.DecisionPoint current = ownerobject(c);
Object item = param(1);
Conveyor conveyor = param(2);
Conveyor.Item conveyorItem;
if(objectexists(item))
{
    conveyorItem = conveyor.itemData[item];
}