Beginner: "An exception of type 'System.NullReferenceException' occurred in RPG.Core.dll but was not handled in user code"

Question

This may be something fairly trivial, but I've decided to jump feet first into this and don't quite understand how to handle this.

   private void OnBattleUpdate(object sender, ElapsedEventArgs e)
    {
        if (monster.IsAlive)
        {
            ActionResult result = monster.GetAttackResult();
            messages.Add(result.LookMessage);
            state.Character.CauseDamage(result.HealthChange);
            if (state.Character.Health <= 0)
            {
                messages.Add("You are dead.");
            }

            int playerDamage = state.Character.GetDamage().HealthChange;
            messages.Add(String.Format("You swing your sword at {0} and cause {1} damage", monster.Name, playerDamage));
            monster.CauseDamage(playerDamage);
            messages.Add(String.Format("Health : {0}/{1}", state.Character.Health, state.Character.MaxHealth));
        }
        else
        {
            messages.Add(String.Format("{0} is dead.", monster.Name));
            state.Character.Gold += monster.Gold;
            mapManager.RemoveMonster(monster, mapManager.MapName);
            battleTimer.Stop();
        }
    }

The code to handle "IsAlive"

    private bool _isAlive;
    public bool IsAlive
    {
        get { return _isAlive; }
        set
        {
            if (Health > 0)
            {
                _isAlive = true;
            }
            else
            {
                _isAlive = false;
            }
        }
    }

Am I just approaching this the wrong way? Any tips that you all can provide would be extremely helpful :)