单击屏幕时，角色无法移动。

NullReferenceException: Object reference not set to an instance of an object
HeroMove.Update () (at Assets/HeroMove.cs:13)



void Update () {
		if(Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);//第13行
            RaycastHit hit;
            if(Physics.Raycast(ray,out hit))
            {
                print(hit.point);
                agent.SetDestination(hit.point);
            }
        }
	}

//hero已经添加了navmeshagent并已经烘培导航网格