Need help with an issue regarding lists (I think).

Hello. I apologize in advance if this is not the correct place to post this question. I recently started learning how to use Unity, and I ran into a problem I can't seem to solve.

For a bit of context, I'm trying to create a Tetris-style game. Rather than randomly generate all pieces, I decided to do what the actual games do and give pieces in sets of 7, just in a random order. I created an empty spawner to hold 2 scripts: one to "hold" the default pieces and a list of the upcoming pieces in the game, and one to actually spawn them. While I was doing this second one, I ran into this problem.

The "UpcomingPiecesManager" script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class UpcomingPiecesManager : MonoBehaviour
 {
     // An array with the pieces that will be generated by default
     public GameObject[] standardPieces;
 
     // A list of the upcoming pieces, which are decided in advance
     private List<GameObject> upcomingPieces = new List<GameObject>();
 
     // Start is called before the first frame update
     void Start()
     {
         // Start with at least 16 pieces
         while (upcomingPieces.Count < 16)
         {
             AddSetOfPieces();
         }
     }
 
     // Update is called once per frame
     void Update()
     {
         // Keep at least 16 pieces generated at a time
         if (upcomingPieces.Count < 16)
         {
             AddSetOfPieces();
         }
     }
 
     // Randomize the order of a given set of pieces
     public GameObject[] RandomizePieceOrder(GameObject[] pieces)
     {
         for (int currentPiece = 0; currentPiece < pieces.Length; currentPiece++)
         {
             int randomIndex = Random.Range(0, pieces.Length);
             GameObject temp = pieces[randomIndex];
             pieces[randomIndex] = pieces[currentPiece];
             pieces[currentPiece] = temp;
         }
         return pieces;
     }
 
     // Add a set of the default pieces in a randomized order
     public void AddSetOfPieces()
     {
         GameObject[] upcomingPiecesAddition = RandomizePieceOrder(standardPieces);
             foreach (var piece in upcomingPiecesAddition)
             {
                 upcomingPieces.Add(piece);
             }
     }
 
     public GameObject GetNextPiece(int index = 0)
     {
         return upcomingPieces[index];
     }
 
     public void DiscardNextPiece()
     {
         upcomingPieces.RemoveAt(0);
     }
 
     public void AddUpcomingPiece(GameObject piece)
     {
         upcomingPieces.Add(piece);
     }
 }

The "PieceSpawner" script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PieceSpawner : MonoBehaviour
 {
     // Start is called before the first frame update
     void Start()
     {
         SpawnNextPiece();
     }
 
     // Update is called once per frame
     void Update()
     {
         
     }
 
     // Spawns the next piece in the list and removes it from it
     public void SpawnNextPiece()
     {
         GameObject pieceToSpawn = gameObject.GetComponent<UpcomingPiecesManager>().GetNextPiece();
         gameObject.GetComponent<UpcomingPiecesManager>().DiscardNextPiece();
         Instantiate(pieceToSpawn, transform.position, Quaternion.identity);
     }
 }

When playing the game, I run into this issue:

 ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

Since the index is 0, it seems to imply the list doesn't contain anything yet. I tried changing "AddSetOfPieces" to show a message whenever a piece is added, and it confirms they are added, so that is working, but the message saying they were added only happens after the error message. That seems to imply to me that the scripts are trying to access the list before the pieces are added. How can I solve this?

I apologize if this is a bit hard to read. As I said, I only just started, but I kind of want to solve this.