Hello.
The problem was already reported and it's fixed. It will be public on next coming 1.6.0 QueueMetrics release.
If an agent is removed and there are queues containing it, now the queues are marked in red in the queue list configuration page. The invalid agents are then marked in red when the queue is edited.
The user should edit the agents associated to the red-marked queue then save it again in order to solve the problem.