public void Play(bool withChildren = true);

var ps = GetComponent<ParticleSystem>();
ps.Play();

  Slider qw;
  public GameObject test2;

    // Use this for initialization
    public void Start ()
    {

qw = GetComponentInParent<Slider>();
    }

    public void Update2s()
    {
        
     Debug.Log("№=" + qw.value);
       
   if ((float)qw.value <= 70)
        {
         
          test2. gameObject.GetComponent<ParticleSystem>().Stop();
        }
        else if ((float)qw.value >= 70)
       
        {
          test2.gameObject.GetComponent<ParticleSystem>().Play();
}
          

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class sl : MonoBehaviour {

   public Slider newVl;
   public GameObject test1;


    // Use this for initialization
    public void Start () {

     newVl.onValueChanged.AddListener(delegate { testq();});

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

    public void testq()
{

Debug.Log("№=" + newVl.value);
   
    }

}

NullReferenceException: Object reference not set to an instance of an object
sl.Start () (at Assets/sl.cs:15)