Destroy instance

Hello, I am making a game where a player has to avoid obstacles. As the borders of the level I use a "box" - like geometry that spawns as a list of gameobjects in a row and translates toward the player by y axis. Player's position is not changing by Y axis. So, as soon as the first box moves away from sight of player (camera), I destroy it, remove at index and it should spawn again as the last in the row. The problem is that the compiler is barking. I have: MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it. The problem line is : MoveSpawnDestroyBlocks method. Before I've implemented StateMachine approach, it worked as a charm. Not anymore.

Please help me with it. What's wrong here..?

The spawning script it here:

using UnityEngine; using Code.States; using System.Collections.Generic;

public class Spawning : MonoBehaviour {

 public GameObject block_PF;
 public GameObject player;
 public GameObject obstacle_PF;
 public GameObject badPU_PF;
 public GameObject goodPU_PF;
 public GameObject collectablePU_PF;

 public List<GameObject> blocks;

 private bool readyToSpawnObstacle = false;
 private bool readyToSpawnBadPU    = false;
 private bool readyToSpawnGoodPU    = false;
 private bool readyToSpawnCollectablePU = false;


 public void BlocksStartSpawn()
 {
     for (int i = 0; i < 5; i++) 
     {
         float y_pos = i * -25;
         
         GameObject clone = Instantiate(block_PF) as GameObject;
         clone.transform.position = new Vector3(0, y_pos, 0);
         
         blocks.Add(clone);
     }
 }
 public void MoveSpawnDestroyBlocks(float blockMoveSpeed_)
 {
     for (int i = 0; i < blocks.Count; i++) 
     {
         blocks[i].transform.Translate(new Vector3(0, blockMoveSpeed_ * Time.deltaTime, 0));
         
         if(blocks[i].transform.position.y >= 25)
         {    
             GameObject clone = Instantiate(block_PF) as GameObject;
             clone.transform.position = new Vector3(0, blocks[i].transform.position.y - 100, 0);
             
             blocks.Add(clone);
             Destroy (blocks[0]);
             blocks.RemoveAt(0);
         }
     }
 }


 public void SpawnDestroyObstacles(float obstacleMoveSpeed_)
 {    
     if(readyToSpawnObstacle)
     {
         float z_rand = Random.Range(-2.0f, 2.0f);
         float rand_rot = Random.Range(0.0f, 180.0f);
         
         GameObject clone = Instantiate(obstacle_PF) as GameObject;
         
         clone.transform.position = new Vector3(0, -100, z_rand);
         clone.transform.rotation = Quaternion.Euler(0, rand_rot,0);
         clone.rigidbody.AddForce(0, obstacleMoveSpeed_, 0, ForceMode.Force);
         DestroyObject(clone, 6.0f);
     }
 }
 public void ObstSpawnCounter(float obstacleSpawnDelay_)    
 {
     float obstacleCounter = 0;

     obstacleCounter += Time.deltaTime;
     
     if(obstacleCounter >= obstacleSpawnDelay_)
     {
         readyToSpawnObstacle = true;
         obstacleCounter = 0;
     }
     else
     {
         readyToSpawnObstacle = false;
     }
     
 }


 public void SpawnDestroyBadPU(float badPUSpeed_)
 {
     if(readyToSpawnBadPU)
     {
         float rand_x_pos = Random.Range(-2.0f, 2.0f);
         float rand_z_pos = Random.Range(-2.0f, 2.0f);
         
         GameObject clone = Instantiate(badPU_PF, new Vector3(rand_x_pos, -125, rand_z_pos), Quaternion.identity) as GameObject;
         clone.rigidbody.AddForce(new Vector3(0, badPUSpeed_, 0), ForceMode.Force);
         clone.transform.Rotate(Time.deltaTime, Time.deltaTime, Time.deltaTime, Space.Self);
         readyToSpawnBadPU = false;
         
         DestroyObject(clone, 8.0f);
     }
 }


 public void SpawnDestroyGoodPU(float goodPUSpeed_)
 {
     if(readyToSpawnGoodPU)
     {
         float rand_x_pos = Random.Range(-2.0f, 2.0f);
         float rand_z_pos = Random.Range(-2.0f, 2.0f);
         
         GameObject clone = Instantiate(goodPU_PF, new Vector3(rand_x_pos, -125, rand_z_pos), Quaternion.identity) as GameObject;
         clone.rigidbody.AddForce(new Vector3(0, goodPUSpeed_, 0), ForceMode.Force);
         clone.transform.Rotate(Time.deltaTime, Time.deltaTime, Time.deltaTime, Space.Self);
         readyToSpawnGoodPU = false;
         
         DestroyObject(clone, 8.0f);
     }
 }


 public void SpawnDestroyCollectablePU(float collectablePUSpeed_)
 {
     if(readyToSpawnCollectablePU)
     {
         float rand_x_pos = Random.Range(-2.0f, 2.0f);
         float rand_z_pos = Random.Range(-2.0f, 2.0f);
         
         GameObject clone = Instantiate(collectablePU_PF, new Vector3(rand_x_pos, -125, rand_z_pos), Quaternion.identity) as GameObject;
         clone.rigidbody.AddForce(new Vector3(0, collectablePUSpeed_, 0), ForceMode.Force);
         clone.transform.Rotate(Time.deltaTime, Time.deltaTime, Time.deltaTime, Space.Self);
         readyToSpawnCollectablePU = false;
         
         DestroyObject(clone, 8.0f);
     }
 }


 public void PowerupsCounter(float badPUSpawnDelay_, float goodPUSpawnDelay_, float collectablePUSpawnDelay_)
 {
     float badPUCounter = 0.0f; 
     float goodPUCounter = 0.0f;
     float collectablePUCounter = 0.0f;

     badPUCounter += Time.deltaTime;
     goodPUCounter += Time.deltaTime;
     collectablePUCounter += Time.deltaTime;

     if(badPUCounter >= badPUSpawnDelay_) 
     {
         readyToSpawnBadPU = true;
         badPUCounter = 0;
     }
     if(goodPUCounter >= goodPUSpawnDelay_) 
     {
         readyToSpawnGoodPU = true;
         goodPUCounter = 0;
     }
     if(collectablePUCounter >= collectablePUSpawnDelay_) 
     {
         readyToSpawnCollectablePU = true;
         collectablePUCounter = 0;
     }
 }

}