Object reference not set to an instance of an object

I am translating a script that generates a hexagonal mesh into C# from java and I ran into this issue with the uvs, it works fine except when I try to assign uvs and I get the above error on the line reading "uvVerts[i] = v * uvRadius;" at line 69. Any help would be appreciated.

 using UnityEngine;
 using System.Collections;
 
 [ExecuteInEditMode]
 [RequireComponent(typeof(MeshFilter))]
 [RequireComponent(typeof(MeshRenderer))]
 [RequireComponent(typeof(MeshCollider))]
 public class HexGrid : MonoBehaviour {
 
     public int columns = 100;
     public int rows = 50;
     public float tileSize = 1.0f;
 
     public Material material;
 
     private Color[] colors;
     private Mesh mesh; // = new Mesh();
 
     private Vector2[] uvVerts; //= new Vector2[6];
     private float uvRadius;
     private Vector2 uvDefault;
 
     public int uvRows = 2;  // Number of rows in the texture
     public int uvCols = 2;  // Number of cols in the texture
 
     // Use this for initialization
     void Start () {

         mesh = new Mesh();
         Vector2[] uvVerts = new Vector2[6];
         float uvRadius = 1.0f / uvCols / 2.0f;
         Vector2 uvDefault = new Vector2(uvRadius, uvRadius);
         renderer.material = material;
         
         BuildMesh();
 
         // Set all the cells to different colors
         for (int i = 0; i < rows; i++) {
             for (var j = 0; j < columns; j++) {
                 SetUV(i,j,i%2,j%2);
             }
         }
     }
     
     public void BuildMesh() {
 
         //mesh.normals = normals;
 
         // Assign our mesh to our filter/renderer/collider
         MeshFilter mesh_filter = GetComponent<MeshFilter>();
         MeshRenderer mesh_renderer = GetComponent<MeshRenderer>();
         MeshCollider mesh_collider = GetComponent<MeshCollider>();
 
         mesh = new Mesh();
         mesh_filter.mesh = mesh;
 
         float vertSpacing = 2.0f * Mathf.Cos(30.0f * Mathf.Deg2Rad) * tileSize;
         float horzSpacing = tileSize + Mathf.Sin(30.0f * Mathf.Deg2Rad) * tileSize;
 
         Vector3 currPos = new Vector3(-columns / 2.0f * horzSpacing, 0.0f, rows / 2.0f * vertSpacing);
 
         //layout vertices of a single hex
         Vector3[] hexVerts = new Vector3[6];
         hexVerts[0] = Vector3.zero;
         Vector3 v = Vector3.right;
 
         for (int i = 0; i < 6; i++) {
             hexVerts[i] = v * tileSize;
             uvVerts[i] = v * uvRadius; //THIS LINE
             v = Quaternion.AngleAxis(60.0f, -Vector3.down) * v;
         }
 
         //create the vertices
         Vector3[] vertices = new Vector3[rows * columns * 6];
         int currVert = 0;
         for (int i = 0; i < columns; i++) {
             for (int j = 0; j < rows; j++) {
                 for (int k = 0; k < hexVerts.Length; k++) {
                     vertices[currVert++] = hexVerts[k] + currPos;
                 }
                 currPos.z-= vertSpacing;
             }
             currPos.x += horzSpacing;
             currPos.z = rows / 2.0f * vertSpacing;
             if (i % 2 == 0) 
                 currPos.z -= vertSpacing / 2.0f;
         }
         mesh.vertices = vertices;
         Debug.Log("vertices done");
 
         //create the triangles
         int current = 0;
         int[] triangles = new int[rows * columns * 4 * 3];
         for (int i = 0; i < columns * rows * 6; i += 6) {
             triangles[current++] = i+0;
             triangles[current++] = i+1;
             triangles[current++] = i+5;
             
             triangles[current++] = i+5;
             triangles[current++] = i+1;
             triangles[current++] = i+4;
             
             triangles[current++] = i+4;
             triangles[current++] = i+1;
             triangles[current++] = i+2;
             
             triangles[current++] = i+2;
             triangles[current++] = i+3;
             triangles[current++] = i+4;
         }
         mesh.triangles = triangles;
 
         // Setup a based set of UV coordinates
         Vector2[] uvs = new Vector2[vertices.Length];
         for (int i = 0; i < vertices.Length; i += 6) {
             for (int j = 0; j < 6; j++) {
                 uvs[i+j] = uvVerts[j] + uvDefault;
             }
         }
 
         mesh.uv = uvs;
         mesh.colors = colors;
         mesh.RecalculateBounds();
         mesh.RecalculateNormals();
 
         
         mesh_filter.mesh = mesh;
         mesh_collider.sharedMesh = mesh;
         Debug.Log ("Done Mesh!");
     }
 
     public void SetUV(int row, int col, int atlasRow, int atlasCol) {
         if (row < 0 || row >= rows) return;
         if (col < 0 || col >= columns) return;
         if (atlasRow < 0 || atlasRow >= uvRows) return;
         if (atlasCol < 0 || atlasCol >= uvCols) return;
 
         Vector2[] uvs = mesh.uv;
 
         int Base = col * rows * 6 + row * 6;
         Vector2 offset = new Vector2(atlasCol * uvRadius * 2.0f + uvRadius, atlasRow * uvRadius * 2.0f + uvRadius);
         for (int i = 0; i < 6; i++) {
             uvs[Base + i] = uvVerts[i] + offset;
         }
         mesh.uv = uvs;
     }
 }