Spawn Object on Set Score

I am making a flappy bird clone for fun but I wanted to see if I could add objects to it for spawning. When I run this code, it tells me the GameObject I am referencing has been destroyed. I don't know why it keeps saying that.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 public class GameController : MonoBehaviour {
 
     public GameObject birdPrefab;
     public GameObject birdPrefab2;
     public Transform birdSpawn;
     public float minTime;
     public float maxTime;
     public float timer;
     public PlayerController PC;
     public Text score;
 
     // Use this for initialization
     void Start ()
     {
 
     }
     
     // Update is called once per frame
     void Update ()
     {
         if (timer <= 0 && PC.isStart == true)
         {
             BirdSpawner();
             BirdSpawner2();
         }
         timer -= Time.deltaTime; // timer = timer - time.deltaTime;
         scoreFunction();
     }
 
     void BirdSpawner()
     {
         Instantiate(birdPrefab, birdSpawn.position, birdSpawn.rotation);
         timer = Random.Range(minTime, maxTime);
        
     }
 
     void BirdSpawner2()
     {
         Instantiate(birdPrefab2, birdSpawn.position, birdSpawn.rotation);
         timer = Random.Range(minTime, maxTime);
 
     }
 
     void scoreFunction()
     {
         score.text = PC.playerScore.ToString();
     }
 
 
     public void ReplayFunction()
     {
         Time.timeScale = 1;
         SceneManager.LoadScene(1);
     }
 
     public void MainMenuButton()
     {
         Time.timeScale = 1;
         SceneManager.LoadScene(0);
     }
 }