老师我在黑暗之光案例中用按键模拟拾取物品添加到背包中时，不能正常添加物品，并且报空指针的错误

NullReferenceException: Object reference not set to an instance of an object
InventoryItemGrid.SetId (Int32 id, Int32 num) (at Assets/Scripts/Custom/InventoryItemGrid.cs:19)
Inventory.GetId (Int32 id) (at Assets/Scripts/Custom/Inventory.cs:65)
Inventory.Update () (at Assets/Scripts/Custom/Inventory.cs:24)
代码如下：

public   int id = 0;
    private  int num = 0;
    private ObjectsInfo.ObjectInfo info = null;
    private UILabel numLabel;

    void Start() {
        numLabel = this.GetComponentInChildren<UILabel>();
    }

    public void SetId(int id ,int num = 1) {
        this.id = id;
         info = ObjectsInfo._instance.GetObjectInfoById(id);
        InventoryItem item = this.GetComponentInChildren<InventoryItem>();
        item.SetIconName(info.icon_name);//这一条是报错跳转的代码
        numLabel.enabled = true;
        this.num = num;
        numLabel.text = num.ToString();
    }


    //清空格子内的物品信息

    public void CleraInfo() {
        id = 0;
        num = 0;
        info = null;
        numLabel.enabled = false;
    }
    public void PlusNumber(int num = 1) {
        this.num += num;
        numLabel.text = this.num.ToString();
    }