Unity2D, can't print a public field on the screen

[Qwertyeerty]

I am doing a 2D platformer using Unity, there is an object that moves by a script (the character). There are diamonds that are put on the level scene, they can be collected by the character. The character's class has a public field that saves the amount of collected diamonds . It's starting value is zero. I managed to make it so that when touching a diamond, this variable increases by 1, and the diamond is destroyed. But I can't display the value of this field on the screen. I tried it like this: I created a canvas, put the text on it, hung the following script on the text:

using UnityEngine;
using UnityEngine.UI;
using System.Collections;
using System.Collections.Generic;
 
public class textScr : MonoBehaviour { 
    private MovingObj _movingObj;
    private Text _diamondText;
 
    void Start()
    {
        _movingObj = GameObject.Find("Square").GetComponent<MovingObj>();
    }
 
    public void Update()
    {
        _diamondText.text = "Diamonds: " + _movingObj.diamondsQuantity.ToString();
    }
 
}

The charachte Throws "NullReferenceE xception: Object reference not set to an instance of an object" on line 17... Maybe, the problem is with the charcter script? It is here:

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

public class MovingObj : MonoBehaviour
{
    [SerializeField] private float speed = 2f; // скорость движения
    [SerializeField] private int lives = 5; // скорость движения
    [SerializeField] private float jumpForce = 15f; // сила прыжка
    public int diamondsQuantity = 0;
    private bool isGrounded = false;

    private ContactFilter2D gh;
    private Rigidbody2D rb;
    private SpriteRenderer sprite;

    private void Awake()
    {
        rb = GetComponent<Rigidbody2D>();
        sprite = GetComponentInChildren<SpriteRenderer>();
    }

    private void OnCollisionEnter2D(Collision2D c)
    {
        collisionWithDiamond(c);
    }

    private void FixedUpdate()
    {
        CheckGround();
    }

    private void Update()
    {
        if (Input.GetButton("Horizontal"))
            Run();
        if (isGrounded && Input.GetButtonDown("Jump"))
            Jump();
    }

    private void Run()
    {
        Vector3 dir = transform.right * Input.GetAxis("Horizontal");

        transform.position = Vector3.MoveTowards(transform.position, transform.position + dir, speed * Time.deltaTime);

        sprite.flipX = dir.x < 0.0f;
    }

    private void Jump()
    {
        rb.AddForce(transform.up * jumpForce, ForceMode2D.Impulse);
    }

    private void collisionWithDiamond(Collision2D d)
    {
        if (d.gameObject.tag == "Diamond") //у всех алмазов тег Diamond, сюда можно записать и тот публичный метод
        {
            Destroy(d.gameObject);
            diamondsQuantity++;
        }
    }

    private void CheckGround()
    {
        Collider2D[] collider = Physics2D.OverlapPointAll(new Vector2 (transform.position.x, transform.position.y - 0.5f));
        isGrounded = collider.Length >= 1;
    }
}

How to fix it?
and yes, I know that this will update the text every second, after fixing I am going to write a public method.