我创建了一个定时器倒计时脚本。当说的时间达到一定的时间，我想示例化和产卵预制。

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class TimerScript : MonoBehaviour
{
    [SerializeField] public GameObject PreFab;

    [SerializeField] private Vector3 spawnPosition;
    float x = 0;

    float y = 0;

    float z = 0;

    public float TimeLeft;
    public bool TimerOn = false;

    public Text TimerTxt;

    void Start()
    {
        
        TimerOn = true;
        
    }

    void Update()
    {
        if (TimerOn)
        {
            if (TimeLeft > 0)
            {
                TimeLeft -= Time.deltaTime;
                updateTimer(TimeLeft);
                
            }
            else
            {
                //Debug.Log("Time is UP!");
                TimeLeft = 0;
                TimerOn = false;
            }

           

        }
        if (TimeLeft <= 19.40)
        {

            Instantiate(PreFab, spawnPosition, Quaternion.identity); 
           
        }
    }

    void updateTimer(float currentTime)
    {
        currentTime += 1;

        float minutes = Mathf.FloorToInt(currentTime / 60);
        float seconds = Mathf.FloorToInt(currentTime % 60);

        TimerTxt.text = string.Format("{0:00}:{1:00}", minutes, seconds);
    }

Unity一直说预制件还没有被分配，但我序列化了它并分配了给定的预制件。我做错了什么？倒计时工作正常，没有问题。