Met error: ArgumentNullException: Value cannot be null.

I have a code: public IEnumerator Download(ResourceModel model,List urls) {

         for(int i=0; i< urls.Count; i++)
         {
             Debug.Log(urls[i]);
             using (UnityWebRequest www = UnityWebRequest.Get(urls[i]))
             {
                 StartCoroutine(ShowProgress(www, model.data[i].dataName));
                 yield return www.SendWebRequest();
                 string path = Path.Combine(Application.persistentDataPath, model.data[i].dataName);
                 Debug.Log(path);
                 File.WriteAllBytes(path, www.downloadHandler.data);
             }
         }
 
     }
 
     IEnumerator ShowProgress(UnityWebRequest www, string fileName)
     {
         _downloadInfoTxt.text = "downloading " + fileName + " file ...";
         while (!www.isDone)
         {
             _loadingBar.fillAmount = www.downloadProgress;
 
             _percenTxt.text = Mathf.RoundToInt(www.downloadProgress * 100) + "%";
 
             
             Debug.Log("Downloaded: " + www.downloadProgress * 100);
             Debug.Log("is Done: " + www.isDone);
 
             yield return new WaitForEndOfFrame();
         }
 
         Debug.Log("=====>> da ra ngoai roi");
 
         _loadingBar.fillAmount = 1;
         _percenTxt.text = "100%";
         _downloadInfoTxt.text = "The " + fileName + " file dowload is completed.";
     }

When i call download IENumrator it work in Unity Editor, but i met the error when run on iOS.

Uploading Crash Report ArgumentNullException: Value cannot be null. Parameter name: unity_self at LoadingView+d_9.MoveNext () [0x00000] in :0 at UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) [0x00000] in :0

(Filename: currently not available on il2cpp Line: -1)

This method to show download progress and saves the file to a folder.

Please, tell me why I met the error and how to fix it. Thanks.