Method doesn't work inside the script where it was created

Question

While using method inside the script where it was declared it is not working and gives NullRefferenceException, but in another script it works. The method im talking about is: RestartLevel

Here is the code of first script

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class LevelInfo : MonoBehaviour {

    public GameObject endGameMenu;
    [SerializeField] GameObject holePref;
    [SerializeField] GameObject holePrefCollider;

    Ball ball;
    TurnOffGroundCollider holeUpCollider;
    Hole hole;
    Score score;
    bool menuIsInstantiated = false;

    private void Start()
    {
        ball = FindObjectOfType<Ball>();
        score = FindObjectOfType<Score>();
        InstanciateHole();
    }

    private void Update()
    {
            if (ball.transform.position.x > -4.9f &&
            ball.GetComponent<Rigidbody2D>().velocity == new Vector2(0, 0))
            {
                    Debug.Log("Condition 1 done");
                    if (GameObject.FindGameObjectsWithTag("Menu").Length < 1)
                    {
                        Debug.Log("Condition 2 done");
                        if (menuIsInstantiated == false)
                        {
                            Debug.Log("Condition 3 done");
                            GameObject menu = InstantiateMenu();
                            menu.transform.SetParent(GameObject.FindGameObjectWithTag("Canvas").transform, false);
                            menuIsInstantiated = true;
                        }

                    }     
            }
    }

    public void RestartLevel()
    {
        ball.DefineStartPosition();
        ball.isAbleToPush = true;
        InstanciateHole();
    }

    public GameObject InstantiateMenu()
    {
        Vector3 menuPosition = new Vector3(0, 0, 0);
        GameObject a = Instantiate(endGameMenu, menuPosition, Quaternion.identity);
        return a;
    }

    public void RestartGame()
    {
        //score.ResetScore();
        ball.DefineStartPosition();
        ball.isAbleToPush = true;
        menuIsInstantiated = false;
    }

    public void NewGame()
    {
        Destroy(GameObject.FindGameObjectWithTag("Menu"));
        menuIsInstantiated = false;
        RestartLevel();

    }

    public Vector3 DefineHolePosition()
    {
        float positionX = Random.Range(-1, 8);
        float positionY = 0;
        float positionZ = 0;
        Vector3 position = new Vector3(positionX, positionY, positionZ);
        return position;
    }
    public void InstanciateHole()
    {
        Vector3 position = DefineHolePosition();
        Instantiate(holePref, position, Quaternion.identity);
        Vector3 difference = new Vector3(0, -0.61f, 0);
        Vector3 positionCollider = position + difference;
        Instantiate(holePrefCollider, positionCollider, Quaternion.identity);
    }
}

Here is a code of another script where this method works

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Hole : MonoBehaviour {

    //[SerializeField] GameObject hole;
    Score objectScore;
    LevelInfo levelInfo;
    public bool holeIsHit = false;


    private void Start()
    {
        objectScore = FindObjectOfType<Score>();
        levelInfo = FindObjectOfType<LevelInfo>();
    }


    private void OnCollisionEnter2D(Collision2D collision)
    {
        holeIsHit = true;
    }
    private void Update()
    {
        if(holeIsHit == true)
        {
            objectScore.AddScore();
            Destroy(gameObject);
            levelInfo.RestartLevel();
            holeIsHit = false;
        }
    } 
}

Here is the script which variables are changed in the RestartLevel

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Ball : MonoBehaviour {

    [SerializeField] float max = 10;
    [SerializeField] float current = 0;

    private Rigidbody2D rb;
    Vector2 ballVelocity = new Vector2();

    public Trajectory trajectory;
    public Vector2 ballStartPosition = new Vector2(-5, 0);

    public bool isAbleToPush = true;

    void Start ()
    {
        rb = GetComponent<Rigidbody2D>() as Rigidbody2D;
        DefineStartPosition();
    }

    void Update()
    {
        ThrowTheBall();
    }

    private void ThrowTheBall()
    {
        if (isAbleToPush)
        {
            if (Input.GetButtonDown("Fire1"))
            {
                StartCoroutine("changeVelocity");

            }

            if (Input.GetButtonUp("Fire1") || current >= max)
            {
                StopCoroutine("changeVelocity");
                rb.velocity = ballVelocity;
                current = 0;
                isAbleToPush = false;
                ballVelocity.Set(0, 0);

            }
        }
    }

    IEnumerator changeVelocity()
    {
        while (true)
        {
            current += 0.04f;
            ballVelocity.Set(current, current);
            trajectory.ShowTrajectory(transform.position, ballVelocity);

            yield return new WaitForSeconds(0.01f);
        }
    }

    public void DefineStartPosition()
    {
        transform.position = ballStartPosition;
        rb.velocity = ballVelocity;
    }
}