repeated nullreferenceexception errors

Trying to make a BSP dungeon, keep getting nullreferenceexceptions over and over again, when I fix one another pops up a few lines later. Can't figure this one out tho..

using UnityEngine; using System.Collections;

 public class BSP_Dungeon
 {
     public BSP_Node root;
     public BSP_Node[] leafNodes;
     public BSP_Node[] oldNodes;
  
   
     public void doSplits()
     {
         this.clearRooms(this.root);
         this.oldNodes = this.leafNodes;
         if(this.oldNodes == null)
             this.oldNodes[0] = new BSP_Node(); //attempted fix...nothing is working ;_;
         this.leafNodes = null;
         for(int i=0; i < this.oldNodes.Length; i++) //original error figure its because of oldnodes is null but I try and put something in oldNodes ^^^^^^ and still throws exception...
         {
             BSP_Node n1 = new BSP_Node();
             BSP_Node n2 = new BSP_Node();
             n1.parent = this.oldNodes[i];
             n2.parent = this.oldNodes[i];
             n1.sister = n2;
             n2.sister = n1;
             int target = Random.Range(this.minSplit, this.maxSplit);
             target = target / 100;
             Point start = new Point(0,0);
             Point end = new Point(0,0);
   

 using UnityEngine;
 using System.Collections;
 
 public class BSP_Node
 {
     public BSP_Node currentNode;
     public BSP_Node root;
     public BSP_Node parent;
     public BSP_Node sister;
     public BSP_Node[] rightChild;
     public BSP_Node[] leftChild;
     
     public BSP_Node()
     {
         this.currentNode = null;
         this.root = null;
         this.parent = null;
         this.sister = null;
     }
 }

I really can't figure this stuff out. Looked at probably 2 dozen of these error's threads and none seem to fit..most are easy fixes like check if it's null but I know it's null but not sure how to fix it..Any help appreciated