how do i fix this error

I am following this tut and I cant figure out were this error is coming from.

  hears the tut
     http://studentgamedev.blogspot.no/2013/08/VoxelTutP2.html
 
     the error
     NullReferenceException
     UnityEngine.Mesh.Clear () (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/Graphics.cs:609)
     PolygonGenerator.UpdateMesh () (at Assets/PolygonGenerator.cs:84)
     PolygonGenerator.Start () (at Assets/PolygonGenerator.cs:33)

hears the script

 using UnityEngine;
 using System.Collections.Generic;
 
 public class PolygonGenerator : MonoBehaviour 
 {
     // This first list contains every vertex of the mesh that we are going to render
     public List<Vector3> newVertices = new List<Vector3>();
     
     // The triangles tell Unity how to build each section of the mesh joining
     // the vertices
     public List<int> newTriangles = new List<int>();
     
     // The UV list is unimportant right now but it tells Unity how the texture is
     // aligned on each polygon
     public List<Vector2> newUV = new List<Vector2>();
     
     
     // A mesh is made up of the vertices, triangles and UVs we are going to define,
     // after we make them up we'll save them as this mesh
     private Mesh mesh;
 
     private float tUnit = 0.25f;
     private Vector2 tStone = new Vector2 (0, 0);
     private Vector2 tGrass = new Vector2 (0, 1);
 
     private int squareCount;
 
     public byte[,] blocks;
 
     void Start () {
         GenTerrain();
         BuildMesh();
         UpdateMesh();
 
         mesh = GetComponent<MeshFilter> ().mesh;
         
         float x = transform.position.x;
         float y = transform.position.y;
         float z = transform.position.z;
         
         newVertices.Add( new Vector3 (x  , y  , z ));
         newVertices.Add( new Vector3 (x + 1 , y  , z ));
         newVertices.Add( new Vector3 (x + 1 , y-1 , z ));
         newVertices.Add( new Vector3 (x  , y-1 , z ));
         
         newTriangles.Add(0);
         newTriangles.Add(1);
         newTriangles.Add(3);
         newTriangles.Add(1);
         newTriangles.Add(2);
         newTriangles.Add(3);
         
         newUV.Add(new Vector2 (tUnit * tStone.x, tUnit * tStone.y + tUnit));
         newUV.Add(new Vector2 (tUnit * tStone.x + tUnit, tUnit * tStone.y + tUnit));
         newUV.Add(new Vector2 (tUnit * tStone.x + tUnit, tUnit * tStone.y));
         newUV.Add(new Vector2 (tUnit * tStone.x, tUnit * tStone.y));
 
     }
 
 
 
     void GenSquare(int x, int y, Vector2 texture){
         newVertices.Add( new Vector3 (x  , y  , 0 ));
         newVertices.Add( new Vector3 (x + 1 , y  , 0 ));
         newVertices.Add( new Vector3 (x + 1 , y-1 , 0 ));
         newVertices.Add( new Vector3 (x  , y-1 , 0 ));
         
         newTriangles.Add(squareCount*4);
         newTriangles.Add((squareCount*4)+1);
         newTriangles.Add((squareCount*4)+3);
         newTriangles.Add((squareCount*4)+1);
         newTriangles.Add((squareCount*4)+2);
         newTriangles.Add((squareCount*4)+3);
         
         newUV.Add(new Vector2 (tUnit * texture.x, tUnit * texture.y + tUnit));
         newUV.Add(new Vector2 (tUnit*texture.x+tUnit, tUnit*texture.y+tUnit));
         newUV.Add(new Vector2 (tUnit * texture.x + tUnit, tUnit * texture.y));
         newUV.Add(new Vector2 (tUnit * texture.x, tUnit * texture.y));
         
         squareCount++;
     }
 
     void UpdateMesh () {
         mesh.Clear ();
         mesh.vertices = newVertices.ToArray();
         mesh.triangles = newTriangles.ToArray();
         mesh.uv = newUV.ToArray();
         mesh.Optimize ();
         mesh.RecalculateNormals ();
         
         squareCount=0;
         newVertices.Clear();
         newTriangles.Clear();
         newUV.Clear();
         
     }
 
     void GenTerrain(){
         blocks=new byte[10,10];
         
         for(int px=0;px<blocks.GetLength(0);px++){
             for(int py=0;py<blocks.GetLength(1);py++){
                 if(py==5){
                     blocks[px,py]=2;
                 } else if(py<5){
                     blocks[px,py]=1;
                 }
             }
         }
     }
 
     void BuildMesh(){
         for(int px=0;px<blocks.GetLength(0);px++){
             for(int py=0;py<blocks.GetLength(1);py++){
                 
                 if(blocks[px,py]==1){
                     GenSquare(px,py,tStone);
                 } else if(blocks[px,py]==2){
                     GenSquare(px,py,tGrass);
                 }
                 
             }
         }
     }
 
 
 }