NullReferenceException: Object reference not set to an instance of an object

Hello,

I have a problem with object pooler function since i adding some rules for prefab spawning (min spawn & max spawn). When i run the code, It doesnt work and keeps prompt me errors: NullReferenceException: Object reference not set to an instance of an object objectpooler.GetPooledObject () (at Assets/Script/objectpooler.cs:66) spawncontroller.Update () (at Assets/Script/spawncontroller.cs:56)

Here's the code: (before this code, i have been trying many ways, such as build a function class for control the spawn rules and call it in "objectpooler" class but its also give me the same errors)

     public GameObject GetPooledObject()
     {
         for (int i = 0; i < pooledObjects.Count; i++)
         {
             if (!pooledObjects[i].activeInHierarchy)
             {
                 if (pooledObjects[i].GetComponent<PrefabStat>().max > globalvariables.freq[pooledObjects[i].GetComponent<PrefabStat>().index])
                 {
                     continue;
                 }
 
 
                 // globalvariables.freq[pooledObjects[i].GetComponent<PrefabStat>().index] += 1;
 
                 return pooledObjects[i];
 
             }
         }
 
         do {
 
             pooledObjectindex = Random.Range(0, pooledObject.Length);
 
 
         //    Debug.Log(pooledObjectindex);
         //      checkresult = check.freqcheck(pooledObjectindex);
         //     Debug.Log(checkresult);
         //    if (checkresult == true)
 
         }
         while (pooledObject[pooledObjectindex].GetComponent<PrefabStat>().max > globalvariables.freq[pooledObject[pooledObjectindex].GetComponent<PrefabStat>().index]);
 
 
         GameObject obj = (GameObject)Instantiate(pooledObject[pooledObjectindex]);
             obj.SetActive(false);
             pooledObjects.Add(obj);
             globalvariables.freq[pooledObject[pooledObjectindex].GetComponent<PrefabStat>().index] += 1;
             return obj;
 
     }

and the code i use for calling it

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

     Quaternion spawnRotation = Quaternion.identity;


     if (transform.position.x < groundspawnpoint.position.x)
     {
         //new ground position
         transform.position = new Vector3(transform.position.x + groundwidth + distancebetween, transform.position.y, transform.position.z);
       
     
         //spawn ground
   
         GameObject groundOBJ = theObjectPool.GetPooledObject();
         groundOBJ.transform.position = transform.position;
         groundOBJ.transform.rotation = transform.rotation;
         groundOBJ.SetActive(true);

         //ground width
         groundwidth = groundOBJ.GetComponent<BoxCollider2D>().size.x;}}

It keeps happen, why i can't called some function or get some variables from others class in the function that i use to be called in others function

Thanks In Advance ^^, sorry about my english.