Can't add GameObjects to a List

I am writing a dungeon generator. Its in very early stages. I got it to detect 1 exit node and spawn a hall tile and a room tile beyond that. An exit node is an empty GameObject. Now I want to make a list to store more then one exit node. I can then later iterate through all the nodes and spawn halls and rooms. I am stuck at this error:

NullReferenceException: Object reference not set to an instance of an object Dungeon.AddNodes (UnityEngine.Transform nodes) (at Assets/Dungeon.cs:47) Dungeon.FindNodes () (at Assets/Dungeon.cs:41) Dungeon.Start () (at Assets/Dungeon.cs:19)

Here is the code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Dungeon : MonoBehaviour {
     
     public GameObject start;  //holds the start tile
     public GameObject player; //holds the player pawn
     public GameObject halls;  //holds the hall tiles, will be changed to an array later.
     public GameObject rooms;  //holds the room tiles, will be changed to an array later.
     
     
     private List<Transform> exitNodes; //holds the current nodes, will be updated after each iteration of room generation.
     
     
     
     // Use this for initialization
     void Start () {
         CreateStart();
         FindNodes();        
         SpawnDungeon();
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     private void CreateStart()
     {
         //creates the starting tile and player pawn.
         GameObject go = Instantiate(start, Vector3.zero, Quaternion.identity) as GameObject; 
         GameObject pc = Instantiate(player, Vector3.up, Quaternion.identity) as GameObject;
     }
     
 
     
     private void FindNodes()
     {
         //Finds all nodes tagged for the north side of the tile, indicating a positive x translation.
         GameObject[] node = GameObject.FindGameObjectsWithTag("N_exit");
         foreach(GameObject temp in node)
         {
             // adds the nodes to exitNode list, not working...
             exitNodes.Add(temp.transform);
         }
     }
     
     
     
     private void SpawnDungeon()
     {
         // Vector3.zero will become the location of the current node.
         GameObject spawn = Instantiate(halls, Vector3.zero, Quaternion.identity) as GameObject;
         //translate to the proper offset.
         spawn.transform.Translate(2.5f,0,0);
         //Vector3.zero will become the location of the current node.
         spawn = Instantiate(rooms, Vector3.zero, Quaternion.identity) as GameObject;
         //translate to the proper offset.
         spawn.transform.Translate(10,0,0);
     }
 }