Null Reference Exception Assigning Vertices to Mesh

I am getting a Null Reference Exception during runtime on the line where I assign the vertices to the mesh here is the code...

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.IO;
 
 [RequireComponent (typeof (MeshCollider))]
 [RequireComponent (typeof (MeshFilter))]
 [RequireComponent (typeof (MeshRenderer))]
 public class Geometric: MonoBehaviour
 {
     public float blockheight = 1.0f;    
     public Material blockmaterial;
     public PhysicMaterial physical;
     public MeshRenderer meshrenderer;        
     public GameObject newbox;
     public int t = 0;    
     void Update()    
     {    
         t++;    
         if(t == 5)
             CreateBox();
     }
 
     public void CreateBox()
     {
         Vector3 p1 = new Vector3(0,0,0);
         Vector3 p2 = new Vector3(4,0,0);
         MeshFilter meshFilter = GetComponent<MeshFilter>();        
         Mesh newmesh = meshFilter.sharedMesh;        
         //newmesh.Clear();    
 
         //FIND SOME VERTICES        
         //wherever p is located the z plane will be placed realtive to the coordinate        
         Vector3 topLeftFront = p1;
         Vector3 topRightFront = p2;
         Vector3 topLeftBack = p1;
         Vector3 topRightBack = p2;
 
         topRightFront.z =.5f;
         topLeftFront.z = .5f;
         topLeftBack.z = -.5f;
         topRightBack.z = -.5f;    
 
         //copy some coordinates to alter from previois vertices assignments        
         Vector3 bottomLeftFront = topLeftFront;        
         Vector3 bottomRightFront = topRightFront;
         Vector3 bottomLeftBack = bottomLeftFront;
         Vector3 bottomRightBack = bottomRightFront;
 
         bottomLeftBack.y = bottomLeftFront.y = bottomRightBack.y = bottomRightFront.y = -1*blockheight;        
 
         //assign the vertex positions to the mesh vertices array        
         newmesh.vertices = new Vector3[]{topLeftFront,topRightFront,topLeftBack,topRightBack,bottomLeftFront,
                            bottomRightFront,bottomLeftBack,bottomRightBack};
 
         //assign uvs based on vertices in array?        
         Vector2[] uvs = new Vector2[newmesh.vertices.Length];    
         for(int i=0; i < newmesh.vertices.Length; i++)
         {
             Debug.Log(newmesh.vertices[i]);    
             uvs[i] = new Vector2(newmesh.vertices[i].x, newmesh.vertices[i].z);    
         }
         newmesh.uv = uvs;
 
         //group sets of 3 vertices into triagles endpoints        
         newmesh.triangles = new int[]{0,1,5,1,5,6,0,2,3,2,3,1,2,0,4,0,4,6,2,3,7,3,7,4,3,1,5,1,5,7,4,6,7,6,7,5};
 
         //box the mesh                
         newmesh.RecalculateNormals();
         newmesh.RecalculateBounds();
         newmesh.Optimize();
     }            
 }