Object reference not set to an instance of an object

I know this is a common error and I tried to find a solution reading the answers on others having this problem but I have not yet found it. And since this is an assignement wich needs to be done tommorow I am asking my specific problem here. So first everything worked well but I had to convert all my code so that it used inheritance. When I converted all the code everything worked fine except my local multiplayer mode where players on the same computer can play against eachother. The error I get is : Object reference not set to an instance of an object. The scripts wich may contain the problems :

-The snowball script

 using UnityEngine;
 using System.Collections;
 
 
 public class Snowball : MonoBehaviour {
 
     public float ballSpeed;
     public GameObject snowBallEffect;
     private Rigidbody2D theRB;
 
     void Start() {
         theRB = GetComponent<Rigidbody2D>();
      
     }
     void Update() {
         //this iis for that the snwobal will move.
         //and that the snowball move as fast as the speed that is set 
         //for the snowball.
         theRB.velocity = new Vector2(ballSpeed * transform.localScale.x, 0);
 
     }
 
 
 }

-The snowballplayer script (wich is on the snowball that the player uses)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class SnowballPlayer : Snowball {
     //this is for when the snowball you throwed hits a enemy
     //that he will lose a life.
     void OnTriggerEnter2D(Collider2D other)
     {
         if (other.tag == "Enemy")
         {
             other.gameObject.GetComponent<Enemy>().HurtEnemy();
         }
 
         //this one is for when player 2 hits player 1
         //player 1 will lose 1 life.
         else if (other.name == "player1")
         {
             other.gameObject.GetComponent<GameManager>().HurtP1();
         }
 
         //this one is for when player 1 hits player 2
         //player 2 will lose 1 life.
         else if (other.name == "player2")
         {
             other.gameObject.GetComponent<GameManager>().HurtP2();
         }
         Instantiate(snowBallEffect, transform.position, transform.rotation);
         Destroy(gameObject);
     }
 }

-The gamemaneger script (wich is being refferenced to for hurting the other player)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class GameManager : MonoBehaviour {
 
     //this script is for the 1v1.
 
     public GameObject player1;
     public GameObject player2;
 
     public int P1Life;
     public int P2Life;
 
     public GameObject p1Wins;
     public GameObject p2Wins;
 
     public GameObject[] p1Sticks;
     public GameObject[] p2Sticks;
 
     public AudioSource hurtSound;
 
     public string mainMenu;
 
     void Update()
     {
         //when player 1 has no lifes anymore he lose.
         //This will show that player 2 to wins. 
         if (P1Life <= 0)
         {
             player1.SetActive(false);
             p2Wins.SetActive(true);
         }
         //when player 2 has no lifes anymore he lose.
         //This will show that player 1 to wins. 
         if (P2Life <= 0)
         {
             player2.SetActive(false);
             p1Wins.SetActive(true);
         }
         //this will show the screen for going back or replay the game.
         if (Input.GetKeyDown(KeyCode.R))
         {
             SceneManager.LoadScene(SceneManager.GetActiveScene().name);
         }
         if (Input.GetKeyDown(KeyCode.Escape))
         {
             SceneManager.LoadScene(mainMenu);
         }
     }
     public void HurtP1()
     {
         //this is for when the player 1 gets hit he will lose a life.
         P1Life -= 1;
         //there are candysticks that represent the lifes of the player 1.
         //This will delete 1 of them when he gets hit.
         for(int i = 0; i < p1Sticks.Length; i++)
         {
             if(P1Life > i)
             {
                 p1Sticks[i].SetActive(true);
             }
             else
             {
                 p1Sticks[i].SetActive(false);
             }
         }
 
         hurtSound.Play();
     }
 
     public void HurtP2()
     {
         //this is for when the player 2 gets hit he will lose a life.
         P2Life -= 1;
         //there are candysticks that represent the lifes of the player 2.
         //This will delete 1 of them when he gets hit.
         for (int i = 0; i < p2Sticks.Length; i++)
         {
             if (P2Life > i)
             {
                 p2Sticks[i].SetActive(true);
             }
             else
             {
                 p2Sticks[i].SetActive(false);
             }
         }
         hurtSound.Play();
     }
    
 }
 

A big thank you to all that try to help :)