AssetBundle : Can anybody tell what's wrong in my code

This is my code. What I need to do is call ReturningRequested Function to work from another script. When I access bundle in returningRequested and call it from start() it works.

When I call it from my script that has been assigned to an emptyobject in UI, it shows

NullReferenceException: Object reference not set to an instance of an object ServerDownloadManager.ReturningRequested () (at Assets/ServerDownloadManager.cs:41) SceneValues.InstantaniateLizardexamplewithanimationcontrols (Int32 storynumber) (at Assets/Scripts/SceneValues.cs:190) SceneValues.Start () (at Assets/Scripts/SceneValues.cs:65)

Please do guide me should I create a new instance and call it OR Should I find it with findobjectoftype. From ReturningRequested() I am planning to return bundle.loadasset() in the future.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ServerDownloadManager : MonoBehaviour {
 
     public string bundleUrl;
 
 
     static AssetBundle bundle;
     // Use this for initialization
     IEnumerator Start () {
 
         WWW www = new WWW(bundleUrl);
         yield return www;
 
         if(www.error!= null){
             throw new System.Exception("There is an error" + www.error);
         }
 
         bundle = www.assetBundle;
         print(bundle.GetAllAssetNames()[0]+ "Asset Name1");
     //    Instantiate(bundle.LoadAsset("Lizard"));
 
     //    ReturningRequested("Hello");
 
     }
 
 
     public static void ReturningRequested(){
 
         print(bundle.GetAllAssetNames()[0]+ "Asset Name2");
     }
 }