"I am getting a null reference exception error for Pathfollowertest.instance.centerslot.Add(Cube.transform)

 using UnityEngine;
 using System.Collections;
 
 public class InfiniteObjectManager : MonoBehaviour 
 { 
     public Rigidbody cube;
     public static InfiniteObjectManager instance;
     
     Void Start() 
     { GameObject Cube = Instantiate(cube, transform.position, transform.rotation); 
     Pathfollowertest.instance.centerslot.Add(Cube.transform);
         
     } 
 }
 public class Pathfollowertest : MonoBehaviour 
 {
     public static Pathfollowertest instance;
     public List centerslot;    
 }