NullReferenceException: Object reference not set to an instance of an object

Hey! I searched in this forum the solving but it seems I'm a unique case. I wrote a script and I want to attach this script to more than one game object. Specifically I have a Chop script and I attached it to each 'Tree' game objects. It working really well but I get an error: NullReferenceException: Object reference not set to an instance of an object. I don't know why I get this error because the script works well.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Chop : MonoBehaviour {
 
     public Slider remainSlider;
     public Text resourceText;
     public Text remainingText;
     public GameObject canv;
 
     public float timeBetweenChops = 0.5f;
     public float maxRes;
 
     float memRes;
     float timer;
     bool canChop;
 
     void Start()
     {
         memRes = maxRes;
     }
 
     void Update()
     {
         timer += Time.deltaTime;
         if (timer >= timeBetweenChops)
         {
             canChop = true;
         }
     }
 
     void OnTriggerEnter(Collider other)
     {
         if (other.tag == "Player")
         {
             remainSlider.maxValue = maxRes;
             remainSlider.value = memRes;
             resourceText.text = "Tree";
             remainingText.text = memRes.ToString();
             canv.SetActive(true);
         }
     }
 
     void OnTriggerStay(Collider other)
     {
         if (other.tag == "Player")
         {
             if (Input.GetButton("Fire1"))
             {
                 if (canChop)
                 {
                     DoChop();
                 }               
             }
         }
     }
 
     void OnTriggerExit(Collider other)
     {
         if (other.tag == "Player")
         {
             memRes = remainSlider.value;
             remainSlider.maxValue = 0f;
             remainSlider.value = 0f;
             resourceText.text = "";
             remainingText.text = "";
             canv.SetActive(false);
         }
     }
 
     void DoChop()
     {
         if (remainSlider.value != 0)
         {
             timer = 0f;
             canChop = false;
             remainSlider.value--;
             remainingText.text = remainSlider.value.ToString();
         }
     }
 }

The error: NullReferenceException: Object reference not set to an instance of an object Chop.DoChop () (at Assets/Scripts/Chop.cs:74) Chop.OnTriggerStay (UnityEngine.Collider other) (at Assets/Scripts/Chop.cs:53)

This is my first game project what I not do based on the tutorials, so it is possible that the code is bad. Please help!