How can I get the last element of a list of messages in the output when selecting them as variable for an evaluator. I need to get the final message of an agent, so I would do `output.messages[-1]` but that does not work and is always undefined. Manually doing `output.messages[5]` works, but that is of course not feasible since we don’t know the amount of message the agent is going to produce ahead of time.
