Error Object reference not set to an instance of an object!!! Help, I am new!

I'm trying to make a 2D game where there are multiple brutes on the map at the same time. Here are my errors that I am getting: Upon starting the game: NullReferenceException: Object reference not set to an instance of an object bruteController.Start () (at Assets/Scripts/bruteController.cs:23)

My three scripts that are used for this are:

LevelManager: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class LevelManager : MonoBehaviour {

 //Adds a defineable delay to the attack
 public float playerAtAnLength;

 //Says whether or not the player can attack
 private bool playerCanAttack;

 //Use this to reference the playerController script
 public playerController thePlayer;

 //Use this to reference the bruteController script
 public bruteController theBrute;

 public bruteDeath theBruteDeath;

 //Used to store the brutes last position.
 public Vector3 bruteLastPosition;

 //Adds a defineable delay to the animation
 public float playerAtAnDelay;

 //Adds a defineable delay to the actual attack
 public float playerAcAtDelay;

 //Tracks the players velocity
 public float playerVelocity;

 //Allows you to define the amount that the attack should be moved left
 public double moveLeftDist;

 //Player Actual Attack Remove Time is the time that it takes for the attack collider to be removed
 public float playerAcAtReTi;

 //Allows me to set the object which would be spawned when the player is attacking
 public GameObject playerActualAttackCollider;

 //Is true if the last move was right
 public bool lastMoveRight;

 //Allows the conversion of the players position to a double
 private double playerTransformXDouble;

 //Allows you to set to players attack x location when looking left
 private Vector3 playerLocationLeft;

 //Allows you to define the players x position moved to the left
 private float playerAttackMovedLeft;

 // Use this for initialization
 void Start () {
     //Defines what thePlayer is in the playerController script
     thePlayer = FindObjectOfType<playerController>();
     theBruteDeath = FindObjectOfType<bruteDeath>();
     theBrute = FindObjectOfType<bruteController>();
     List<GameObject> Brute = new List<GameObject>();
     //Starts it out so that the player can attack
     playerCanAttack = true;

 }
 
 // Update is called once per frame
 void Update () {

     //
     //
     //All needed to flip the player attack when moving left
     //
     //
     //theBruteDeath = FindObjectOfType<bruteDeath>();
     //theBrute = FindObjectOfType<bruteController>();
     //Sets the playerVelocity to the live playerVelocity
     playerVelocity = thePlayer.playerVelocity;
     //Transfers the lastMoveRight variable to this script
     lastMoveRight = thePlayer.lastMoveRight;
     //Converts the position to a double
     playerTransformXDouble = thePlayer.transform.position.x;
     //Converts the double to a float
     playerAttackMovedLeft = (float)(playerTransformXDouble - moveLeftDist);
     //Sets the attack to the left of the player by a certain amount
     playerLocationLeft = new Vector3(playerAttackMovedLeft, thePlayer.transform.position.y, thePlayer.transform.position.z);
 }

 //
 //
 //
 //THIS CODE HAPPENS WHEN THE PLAYER ATTACKS OR ATTEMPTS TO ATTACK:
 //
 //
 //

 //Is able to be called from the playerController when the player attacks
 public void playerAttack()
 {
     //Will run if the player can attack
     if(playerCanAttack == true)
     {
         //Calls the playerAttackCo() to start
         StartCoroutine("playerAttackCo");
     }
 }
 //The Co-routine to add the delay into the attack, so you cant spam attack.
 public IEnumerator playerAttackCo()
 {
     //Makes the player not able to spam the attack button
     playerCanAttack = false;
     //Starts both co-routines below
     StartCoroutine("playerAtAnCo");
     //Will run if the player is moving right
     if(playerVelocity >= 0.1)
     {
         StartCoroutine("playerAcAtCoRight");
     }
     //Will run if the player is still
     if(playerVelocity == 0)
     {
         StartCoroutine("playerAcAtCoStill");
     }
     //Will run if the player is moving left
     if(playerVelocity <= -0.1)
     {
         StartCoroutine("playerAcAtCoLeft");
     }
     //Sets the canMove variable in the playerController to false
     thePlayer.GetComponent<playerController>().canMove = false;
     //Adds the actual delay for the number of seconds that playerAttackDelay is defined to.
     yield return new WaitForSeconds(playerAtAnLength);
     //Sets the canMove variable in the playerController to true
     thePlayer.GetComponent<playerController>().canMove = true;
     //When the delay is over, the player can attack again.
     playerCanAttack = true;
 }
 //This Co-routine adds enough delay so that the actual animation is played only once
 public IEnumerator playerAtAnCo()
 {
     //Sets the isAttacking variable in the playerController to true
     thePlayer.GetComponent<playerController>().isAttacking = 1;

     //Adds enough delay so that the animation is displayed
     yield return new WaitForSeconds(playerAtAnDelay);

     //Sets the isAttacking variable in the playerController to false
     thePlayer.GetComponent<playerController>().isAttacking = 0;
 }

 //This Co-routine adds the actual damage part into the attack
 public IEnumerator playerAcAtCoRight()
 {
     //Sets the delay for the actual damage to be given
     yield return new WaitForSeconds(playerAcAtDelay);
     //Spawns the attack radius
     Instantiate(playerActualAttackCollider, thePlayer.transform.position, thePlayer.transform.rotation);
 }
 //Lists all of the brutes?
 public List<bruteController> Brute
 {
     //Gets a brute
     get;
     //Sets the brute as a current brute.
     set;
 }
 public IEnumerator playerAcAtCoLeft()
 {
     //Sets the delay for the actual damage to be given
     yield return new WaitForSeconds(playerAcAtDelay);

     //Still need to flip instantiate transform position
     Instantiate(playerActualAttackCollider, playerLocationLeft, thePlayer.transform.rotation);
 }

 public IEnumerator playerAcAtCoStill()
 {
     //Sets the delay for the actual damage to be given
     yield return new WaitForSeconds(playerAcAtDelay);
     //Will run if the last move was right
     if(lastMoveRight == true)
     {
         //Will run and place the attack radius on the right if the player is facing right
         Instantiate(playerActualAttackCollider, thePlayer.transform.position, thePlayer.transform.rotation);
     }
     //Will run if the last move is left
     if (lastMoveRight == false)
     {
         //Will run and place the attack radius on the left if is the player is facing left
         Instantiate(playerActualAttackCollider, playerLocationLeft, thePlayer.transform.rotation);
     }
 }

}

bruteDeath: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class bruteDeath : MonoBehaviour {

 private bruteHealth theBrute;

 public Sprite deadBrute;

 private Animator myAnim;

 public float timeToEnd;

 public LevelManager theLevelManager;

 public Sprite bruteSprite;

 public Vector3 brutePosition;

 public Vector3 bruteRotation;

 public GameObject deadBody;

 private Vector3 bruteLastPosition;







 //The health of the brute
 public float bruteHealthPoints;

 //References the playerController script
 private playerController thePlayer;

 private float playerAt1Dam;







 // Use this for initialization
 void Start()
 {
     theLevelManager = FindObjectOfType<LevelManager>();
     theBrute = FindObjectOfType<bruteHealth>();
     myAnim = GetComponent<Animator>();
     bruteSprite = GetComponent<Sprite>();



     //Actually finds the object with the playerController script
     thePlayer = FindObjectOfType<playerController>();
     //Sets the player Attack 1 Damage in this script to the defined amount in the playerController
     playerAt1Dam = thePlayer.playerAttack1Dam;
 }

 // Update is called once per frame
 void Update() {
     //Checks to see if the brute has run out of health
     if (bruteHealthPoints <= 0)
     {
         //If the brute has no health, then they are removed from the world. Eventually, add an Instantiate with particales
         bruteAnimDeath();
     }
 }
 public void bruteActualDeath()
 {
     bruteLastPosition = new Vector3(theBrute.transform.position.x, theBrute.transform.position.y, theBrute.transform.position.z);
     addDeadBrute();
 }
 public void bruteAnimDeath()
 {
     myAnim.SetBool("bruteIsDead", true);
 }
 public void bruteDeathAnimDone()
 {
     myAnim.SetBool("bruteDeathAnimDone", true);
     bruteActualDeath();
 }
 public void addDeadBrute()
 {
     Destroy(gameObject);
     Instantiate(deadBody, bruteLastPosition, theBrute.transform.rotation);
 }
 //Kills the brute
 public void Kill()
 {

 }




 public void thePlayerAttack1BruteDamage()
 {
     //Takes the player attack 1 damage amount away from the brutes health.
     bruteHealthPoints = bruteHealthPoints - playerAt1Dam;
 }

}

bruteController: using System.Collections; using System.Collections.Generic; using UnityEngine; [RequireComponent(typeof(bruteDeath))] public class bruteController : MonoBehaviour { //Creates the RigidBody for the brute private Rigidbody2D bruteRigidBody;

 public LevelManager theLevelManager;

 public bruteDeath bruteDeath;
 // Use this for initialization
 void Start () {
     //Gets the needed component for the Rigid Body
     bruteRigidBody = GetComponent<Rigidbody2D>();
     //Defines theLevelManager to find the object with the script attached
     theLevelManager = FindObjectOfType<LevelManager>();

     bruteDeath = GetComponent<bruteDeath>();
     //Registers you to the Level Manager
     theLevelManager.Brute.Add(this);
     //Makes the bruteDeath register as the script
 }
 
 // Update is called once per frame
 void Update () {
     
 }
 //Occurs whenever the goblin enters a trigger collider
 void OnTriggerEnter2D(Collider2D other)
 {
     //Checks to see if the collider was the players attack radius
     if(other.tag == "playerAttack1")
     {
         //Starts the code in the LevelManager if the player attacked the brute
         bruteDeath.thePlayerAttack1BruteDamage();
     }
 }
 public void Kill()
 {
     theLevelManager.Brute.Remove(this);
     bruteDeath.Kill();
 }

}

Please note that line 23 of the bruteController is: theLevelManager.Brute.Add(this);

That code interacts with the LevelManagers List with the get; and set; I am not familiar with lists so an explanation on them would be awesome too! Thanks.