Instantiating a GameObject from scene that changes

So I'm currently trying to instantiate a game object in a scene every few seconds, and that object is constantly changing. I tried linking it as a public variable to the script, but I keep getting null reference exceptions. any idea why this code is unable to spawn the GameObject wall?

 using UnityEngine;
 using System.Collections;
 
 public class BPMController : MonoBehaviour {
     
     public int bpm;
     public GameObject wall;
     public GameObject floor;
     bool wait = false;
     int spawnCounter = 0;
     public int spawnInterval = 4;
 
     //find the wall in the scene
     void Start () {
         wall = GameObject.FindGameObjectWithTag("wall");
     }
 
     void Update () {
 
         //if coroutine isnt running start it
         if (!wait) 
         {
             wait = true;
             StartCoroutine(waitCoRoutine());
         }
         //if interval count is reached, spawn wall
         if(spawnCounter >= spawnInterval)
         {
             spawnWall();
             spawnCounter = 0;
         }
     }
 
     //coroutine to wait a set time and increment counter
     IEnumerator waitCoRoutine(){
         yield return new WaitForSeconds(bpm/60);
         spawnCounter++;
         wait = false;
     }
 
     void spawnWall(){
         //create cloneObject to manipulate
         GameObject wallClone = null;
         //instantiate wallCone as a clone of wall
         wallClone = Instantiate (wall, wall.transform.position, wallClone.transform.rotation) as GameObject;
         //set wallcone as a child of floor;
         wallClone.transform.parent = floor.transform;
     }
 }