Calling a function from an other script

So I want to call the "pickUp()" function from this script:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 
 public class zombieLoot : MonoBehaviour {
 
     int randomNumber;
     Inventory inventory;
 
     void Start () {
         inventory = GetComponent<Inventory> ();
     }
 
     void Update(){
         if (Input.GetButtonDown("F")) {
             pickUp();
         }
     }
 
     public void pickUp(){
                 AddItem(1);
         }
 }

I want to call it here:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 
 public class chase : MonoBehaviour {
 
     zombieLoot zombieloot;
     public Slider healthbar;
     public Transform player;
     static Animator anim;
     public float speed = 0.02f;
 
     void Start(){
         anim = GetComponent<Animator> ();
         zombieloot = GetComponent<zombieLoot> ();
     }
 
     void Update(){
         if (healthbar.value <= 0 && anim.GetBool("isDead")) {
             anim.SetBool ("isAttacking", false);
             zombieloot.pickUp ();
                 }
         }
 }

and it says: NullReferenceException: Object reference not set to an instance of an object

why? In my first script I'm calling the Inventory scripts AddItem function, and it works! I want to call this pickUp function EXACTLY like the AddItem function, and it throws this error. It sees the script, but I don't know what could be wrong here. Help please!