How to stop list from getting shorter ? [Beginner]

I'm currently making a 2D endless runner game in which the player stays still while terrain moves.

I've tried creating a pooling system that has a little problem :

When I run the game, the list which contains my prefabs (10 elements) keeps getting shorter. It continues until it reaches 0. An "Out of Range" message appears in the console. Obviously, I'm counting on my list to make this pooling system work.

Here is my code (very amateurish) :

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Generator : MonoBehaviour {
     
     public List<GameObject> platformList = new List<GameObject>();
 
     //GameObjects
     public GameObject newPlatform = null;
     public GameObject transPlatform = null;
 
     //Booleans
     bool spawnNow = true; //Check if trans platform is gone
 
     public bool triggerHit = false; //When this is true, allow spawn new prefab
 
     //Numbers
     int randomInt; //Random pick
     float gameSpeed = -5f;
 
     //Vector3
     Vector3 offset = new Vector3(25,0,0);
 
     void OnTriggerEnter2D(Collider2D other) {
 
         //The "platforms" tag is the transplatform in this context
         if (other.tag == "Platforms") {
 
             triggerHit = true;
 
         }
     }
     
 
     void Spawn () {
 
         if (spawnNow == true) {
 
             //Pick Random int
             randomInt = Random.Range(1,9);
 
             //Remove designated terrain from list/pool
             platformList.RemoveAt(randomInt);
 
             //Assign designated terrain to new platform variable
             newPlatform = platformList[randomInt];
 
             newPlatform.SetActive(true);
 
             //Set position so it sticks to current terrain
             newPlatform.transform.position = transPlatform.transform.position + offset;
 
 
                 //Set spawnNow to false
                 spawnNow = false;
             }
         }
 
 
     void Despawn () {
 
      while (triggerHit == true) {
 
         if (spawnNow == false) {
 
             //Add old platform back in to the list and deactivate
             transPlatform.SetActive(false);
             platformList.Add(transPlatform);
 
 
                 //Reset platform assignements
                 if (newPlatform.transform.position.x < 0.1f) {
                     
                     transPlatform = newPlatform;
                     
                 }
         }
 
         triggerHit = false;
         spawnNow = true;
 
         }
     }
 
     void SpawnAtStart () {
         
         platformList.RemoveAt(0);
         
         transPlatform = platformList[0];
         
         transPlatform.SetActive(true);
         
         transPlatform.transform.position = Vector3.zero;
     }
 
 
     // Use this for initialization
     void Start () {
         SpawnAtStart();
     }
     
     // Update is called once per frame
     void Update () {
 
         transPlatform.transform.Translate(gameSpeed * Time.deltaTime,0,0);
 
         if (newPlatform != null){
 
         newPlatform.transform.Translate(gameSpeed * Time.deltaTime,0,0);
 
         }
         Spawn();
         Despawn();
     }
 }