EventSystem.Subscribe<ActivityInstance, FinishActivityEventArgs>(GetNextAssigneeDetails, EventPhases.Processed);

private void GetNextAssigneeDetails(ActivityInstance activity, FinishActivityEventArgs args, EventPhases phase)
    {

    string id = args.ActivityFinish.NextAssignee.Id.ToString();
    string title = args.ActivityFinish.NextAssignee.Title.ToString();
    }