Object reference not set to an instance of an object error C#

Just an error that I don't posses the skill to fix. The error is on line 63.

 using UnityEngine;
 using System.Collections;
 
 public class floorbuilder : MonoBehaviour {
 
     //floors
     public GameObject[] fourrooms;
     public GameObject[] threerooms;
     public GameObject[] tworooms;
     public GameObject[] onerooms;
     public GameObject[] startrooms;
     GameObject[] currentfloor4;
     GameObject[] currentfloor3;
     GameObject[] currentfloor2;
     GameObject[] currentfloor1;
     //data containment
     public static int floornumb = 1;
     float doornumb;
     float roomnumb = 4; 
     float fourdoornumb;
     float threedoornumb;
     float twodoornumb;
     float onedoornumb;
     public int Size_Increase_Factor = 3;
     //positional variables
     int xnumb = 0;
     int znumb = 0;
     float snumb = 0;
     float rnumb = 0;
 
     void Start () {
         transform.position = new Vector3 (0, 0, 0);
         buildnextfloor ();
     }
 
     void buildnextfloor(){
 
         //Math determining the number of rooms and doors relitive to the floor level and shape of room positioning
         roomnumb = /*(insert mathy stuff stuff here)*/ (roomnumb + roomnumb) * Size_Increase_Factor;
         fourdoornumb = (Mathf.Round(roomnumb * (Random.Range(0.2f, 0.5f))));
         threedoornumb = (Mathf.Round(roomnumb * (Random.Range(0.1f, 0.4f))));
         twodoornumb = (Mathf.Round(roomnumb * (Random.Range(0.1f, 0.3f))));
         onedoornumb = (Mathf.Round(roomnumb * (Random.Range(0.1f, 0.3f))));
         doornumb = (fourdoornumb * 4) + (threedoornumb * 3) + (twodoornumb * 2) + (onedoornumb);
         roomnumb = (fourdoornumb) + (threedoornumb) + (twodoornumb) + (onedoornumb);
 
         if (doornumb % 2 == 1) {
             onedoornumb ++;
 
             doornumb = (fourdoornumb * 4) + (threedoornumb * 3) + (twodoornumb * 2) + (onedoornumb);
             roomnumb = (fourdoornumb) + (threedoornumb) + (twodoornumb) + (onedoornumb);
         }
 
         Debug.Log ("Number of rooms: " + roomnumb);
         Debug.Log ("Number of doors: " + doornumb);
         Debug.Log ("4doors: " + fourdoornumb); 
         Debug.Log ("3doors: " + threedoornumb);
         Debug.Log ("2doors: " + twodoornumb);
         Debug.Log ("1door: " + onedoornumb);
 
         //Selection of rooms
         for (int i = 0; i < fourdoornumb; i++) {
             currentfloor4[i] = fourrooms[(Random.Range(0, fourrooms.Length))]; 
         }
         for (int i = 0; i < threedoornumb; i++) {
             currentfloor3[i] = threerooms[(Random.Range(0, threerooms.Length))]; 
         }
         for (int i = 0; i < twodoornumb; i++) {
             currentfloor2[i] = tworooms[(Random.Range(0, tworooms.Length))]; 
         }
         for (int i = 0; i < onedoornumb; i++) {
             currentfloor1[i] = onerooms[(Random.Range(0, onerooms.Length))]; 
         }
 
         snumb = (Mathf.Round((Mathf.Sqrt (roomnumb))));
         rnumb = roomnumb - (snumb * snumb);
         snumb = snumb * 100;
         rnumb = rnumb * 100;
 
         //placing of rooms
 
         Instantiate (startrooms[Random.Range(0, startrooms.Length)], transform.position, transform.rotation);
         updatelocation ();
 
         for(int i = 0; i < fourdoornumb; i ++){
             Instantiate(currentfloor4[i], transform.position, transform.rotation);
             updatelocation();
         }
         for(int i = 0; i < threedoornumb; i ++){
             Instantiate(currentfloor3[i], transform.position, transform.rotation);
             updatelocation();
         }
         for(int i = 0; i < twodoornumb; i ++){
             Instantiate(currentfloor2[i], transform.position, transform.rotation);
             updatelocation();
         }
         for(int i = 0; i < onedoornumb; i ++){
             Instantiate(currentfloor1[i], transform.position, transform.rotation);
             updatelocation();
         }
     }
 
     void updatelocation(){
         if(xnumb < snumb){
             xnumb += 100;
         }
         else if (znumb < snumb){
             znumb += 100;
             xnumb = 0;
         }
         else{
             znumb += 100;
         }
 
         transform.position = new Vector3 (xnumb, 0f, znumb);
     }
 }
 

Any help would be awesome.