求助！！！老是说我对象没有实例化（或者实例化对象为空）

以下为代码
public class TankAI : Unit {
public float moveSpeed;
public float attackRange;
public GameObject player;
private Tankweapon tw; //Tankweapon 是坦克的武器发射，单独的一个类
void Start(){
tw = GetComponent<Tankweapon> ();
}
void FixedUpdate () {
float dist= Vector3.Distance (player.transform.position,transform.position);
if (dist > attackRange) {
transform.Translate (Vector3.forward * moveSpeed * Time.deltaTime);
}
transform.LookAt (player.transform.position);
tw.Shoot ();
}
}
错误情况是
NullReferenceException: Object reference not set to an instance of an object
TankAI.FixedUpdate () (at Assets/Scripts/Tank/TankAI.cs:19)
求dalao们帮下忙