NullReferenceException Error on first frame, despite everything functioning correctly

This error started happening suddenly and I can't determine why... On the first frame the game stops with this error:

NullReferenceException: Object reference not set to an instance of an object TurnManager.AddUnit (TacticsMove unit) (at Assets/Scripts/TurnManager.cs:85)
TacticsMove.Init () (at Assets/Scripts/TacticsMove.cs:46)
PlayerMove.Start () (at Assets/Scripts/PlayerMove.cs:11)

referencing line 85 of the TurnManager script (I cut out irrelevant bit):

if (!gameStarted && (playerList.Count == startingPlayers.Length))

But I can't for the life of me figure out which variable is causing the error. Am I declaring the List or Array wrong? The strangest thing is that if I unpause from the error, everything works perfectly. Is it simply an issue of timing? I'm a first time coder figuring things out so any advice would be immensely appreciated. I didn't include the TacticsMove script because I'm assuming the lines referencing it are just pointing to what is calling the function...but I could be very wrong of course

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class TurnManager : MonoBehaviour
 {
     public static List<TacticsMove> playerList = new List<TacticsMove>();
     public static List<TacticsMove> enemyList = new List<TacticsMove>();
     public static Queue<TacticsMove> enemyQueue = new Queue<TacticsMove>();
     public static int playerCount;
     public static bool playerRound = false;
 
     public static GameObject[] startingPlayers;
     public static bool gameStarted = false;
 
     // Start is called before the first frame update
     void Start()
     {
         startingPlayers = GameObject.FindGameObjectsWithTag("Player");
         Debug.Log("THERE ARE " + startingPlayers.Length + " PLAYERS");
     }
 
     public static void AddUnit(TacticsMove unit)
     {
         if (unit.tag == "Player")
         {
             playerList.Add(unit);
             Debug.Log(unit + " ADDED");
 
             if (!gameStarted && (playerList.Count == startingPlayers.Length))
             {
                 InitPlayerRound();
                 gameStarted = true;
                 //Debug.Log("GAME START");
             }
         }
         else
         {
             enemyList.Add(unit);
             Debug.Log(unit + " ADDED");
         }
     }