Null reference and GetComponent

Working on a grenade area damage script and am getting null reference error.

Its organized as follows:

1. -Grenade prefab has a script called ExploGren.cs attached(this is the

   problem script).

2. -Health.cs is attached to enemy

3. -When g is pressed, grenadeThrow.cs instantiates grenade prefab, after 5 seconds, ExploGren.cs is called. ExploGren.cs calls Health.cs and applies damage.

A null reference error is displayed when the ExploGren.cs happens. I initialized the Health.cs right, but im sure i used the wrong way to call the Damage() method. So how do i fix this?

 using UnityEngine;
 using System.Collections;
 
 public class ExploGren : MonoBehaviour 
 {
     public GameObject explosivo;//the explosive particle
     private GameObject insta;// instatntiation of variables for explosion
 
     private float timer = 0.0f;//zero time
     private float timeTD = 5.0f;//time ot detonate
 
     private int exploDamage = 50;//grenade damage
     private float explosiveRadius = 5.0f;//radious of the explosive damage
 
     Collider[] collidersInRange;
     Vector3 grenadeOrigin;
 
     Health damageByGrenadeExplosion;//declare health 
 
     // Use this for initialization
     void Start () 
     {
         damageByGrenadeExplosion = GetComponent<Health>();
         grenadeOrigin = transform.position;
         collidersInRange = Physics.OverlapSphere(grenadeOrigin, explosiveRadius);
 
     }
     
     // Update is called once per frame
     void Update () 
     {
         timer +=1 * Time.deltaTime;//start counting
 
             if(timer >= timeTD)//if timer is equal to detonation, detonate
             {
                 insta = (GameObject)Instantiate(explosivo, transform.position, transform.rotation);//create the explosion at the grenades position
 
                 foreach(Collider col in collidersInRange)
                     {
                 damageByGrenadeExplosion.Damage(exploDamage);//<--problem is here.
                     }
                 //Destroy(insta, .70f);
             }
 
 
     }
 }