Null reference exception

I'm working on this project from a book and following it word for word and I'm still having issues. In my layout script I keep getting an error saying that I have a null reference exception at line at line 135 or ti.transform.parent = tileAnchor; can someone help me out and tell me what I'm missing. I thought I referenced it with:

public Transform tileAnchor;

I apologize if this is easy to fix, I've only used Unity sparingly for a couple of months now and I'm trying to understand how it works.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 [System.Serializable]
 public class TileTex {
     // This class enables us to define various textures for tiles
     public string            str;
     public Texture2D        tex;
 }
 
 public class LayoutTiles : MonoBehaviour {
     static public LayoutTiles S;
 
 
     public TextAsset        roomsText; // The Rooms.xml file
     public string             roomNumber = "0"; // Current room # as a string
     // ^roomNumber as string allows encoding in the XML & rooms 0-F
     public GameObject        tilePrefab; // Prefab for all Tiles            
     public TileTex[]        tileTextures; // A list of named textures for Tiles
 
     public bool ___________;
 
     public PT_XMLReader     roomsXMLR;
     public PT_XMLHashList     roomsXML;
     public Tile[,]            tiles;
     public Transform        tileAnchor;
 
     void Start() {
         S = this;  // Set the Singleton for LayoutTiles
 
         // Make a new GameObject to be the TileAnchor (the parent transform of
         // all Tiles). This keep Tiles tidy in the Hierarchy pane.
         GameObject tAnc = new GameObject("TileAnchor");
         tileAnchor = tAnc.transform;
         // Read the XML
         roomsXMLR = new PT_XMLReader(); // Create a PT_XMLReader
         roomsXMLR.Parse(roomsText.text); // Parse the Rooms.xml file
         roomsXML = roomsXMLR.xml["xml"][0]["room"]; // Pull all the <room>s
 
         // Build the 0th Room
         BuildRoom(roomNumber);
     }
 
     //This is the GetTileTex() method that Tile uses
     public Texture2D GetTileTex(string tStr) {
         // Search though all the tileTextures for the proper string
         foreach (TileTex tTex in tileTextures) {
             if (tTex.str == tStr) {
                 return (tTex.tex);
             }
         }
         // Return null if nothing was found
         return(null);
     }
     // Build a room based on room number. This is an alternative version of
     // BuildRoom that grabs roomXML based on  num.
     public void BuildRoom (string rNumStr) {
         PT_XMLHashtable roomHT = null;
         for (int i=0; i<roomsXML.Count; i++) {
             PT_XMLHashtable ht = roomsXML[i];
             if (ht.att("num") == rNumStr) {
                 roomHT = ht;
                 break;
             }
         }
         if (roomHT == null) {
             Utils.tr("ERROR","LayoutTiles.BuildRoom()","Room not found: "+rNumStr);
             return;
         }
         BuildRoom(roomHT);
     }
 
     // Build a room from a XML <room> entry
     public void BuildRoom (PT_XMLHashtable room) {
         // Get the texture names for the floors and walls from <room> attributes
         string floorTexStr = room.att("floor");
         string wallTexStr = room.att ("wall");
         // Split the room into rows of tiles based on carriage returns in the
         // Rooms.xml file
         string[] roomRows = room.text.Split('\n');
         // Trim tabs from the beginnings of lines. However, we're leaving spaces
         // and underscores to allow for non-rectangular rooms
         for (int i=0; i<roomRows.Length; i++) {
             roomRows[i] = roomRows[i].Trim('\t');
         }
         // Clear the tiles away
         tiles = new Tile[ 100, 100 ];  // Arbitrary max room size is 100x100
 
         // Declare a number of local fields that we will use later
         Tile ti;
         string type, rawType, tileTexStr;
         GameObject go;
         int height;
         float maxY = roomRows.Length - 1;
 
         // These loops scan through each tile of each row of the room
         for (int y = 0; y < roomRows.Length; y++) {
             for (int x = 0; x < roomRows [y].Length; x++) {
                 // Set defaults
                 height = 0;
                 tileTexStr = floorTexStr;
 
                 // Get the character representing the tile
                 type = rawType = roomRows [y] [x].ToString ();
                 switch (rawType) {
                 case " ": //empty space
                 case "_": // empty space
                     // Just skip over empty space 
                     continue;
                 // Skips to the next iteration of the x loop
                 case ".": // default floor
                     // Keep type="."
                     break;
                 case "|": //defualt wall
                     height = 1;
                     break;
                 default:
                     // Anything else will be interpreted as floor
                     type = ".";
                     break;
                 }
 
                 // Set the texture for floor or wall based on <room> attributes
                 if (type == ".") {
                     tileTexStr = floorTexStr;
                 } else if (type == "|") {
                     tileTexStr = wallTexStr;
                 }
 
                 // Instantiate a new TilePrefab
                 go = Instantiate (tilePrefab) as GameObject;
                 ti = go.GetComponent<Tile>();
                 // Set the parent transform to tileAnchor
                 ti.transform.parent = tileAnchor;
                 // Set the position of the tile
                 ti.pos = new Vector3 (x, maxY - y, 0);
                 tiles [x, y] = ti; //Add ti to the tiles 2D Array
 
                 // Set the type, height and texture of the Tile
                 ti.type = type;
                 ti.height = height;
                 ti.tex = tileTexStr;
 
                 // More to come here...
             }
         }
     }
 }