Problem of generation

Hi,
I was trying to create a random map using blocs (a bit like minecraft) for a fps but the error "NullReferenceException" keep showing up.
I looked in other threads and it was written to create a gameobject but mine is a matrix (and I think the position within the matrix). So I don't really know how to do that.
Any help ?

Code (JavaScript):

1. function Start () {
2.  
3. //Donnees de generation
4.     var X_dim = Random.Range(10, 20);
5.     var Z_dim = Random.Range(10, 20);
6.     var Y_dim_min = Random.Range(-5 , 0);
7.     var Y_dim_max = Random.Range(0, 5);
8.     var terrain = new int[X_dim,Z_dim];
9.  
10. //Generation random
11. /*
12.     for (var x=1 ; x<=X_dim ; x++)
13.     {
14.         for (var z=1 ; z<=Z_dim ; z++ )
15.         {
16.             var y = Random.Range(Y_dim_min, Y_dim_max);
17.             terrain[x,z] = y;
18.         }
19.     }
20. */
21.  
22. //Generation programmee
23.     for (var x = 0; x < X_dim ; x++) {
24.         for (var z = 0; z < Z_dim; z++) {
25.             terrain[x,z] = -6;
26.         }
27.     }
28.     terrain == -6;
29.     var Nb_case = X_dim*Z_dim;
30.     var Temp = 1;
31.     x_pos = Random.Range(0 , X_dim);
32.     z_pos = Random.Range(0 , Z_dim);
33.     var x_new_pos;
34.     var z_new_pos;
35.     var y = Random.Range(Y_dim_min, Y_dim_max);
36.     terrain[x_pos,z_pos] = y;
37.     while(Temp < Nb_case)
38.     {
39.         var direction = Random.Range(1, 5);
40.         var high = Random.Range(-1, 2);
41.      
42.         if(direction == 1 && z_pos < Z_dim + 1){z_new_pos = z_pos + 1;}    //Haut
43.         if(direction == 2 && x_pos < X_dim + 1){x_new_pos = x_pos + 1;}    //Droite
44.         if(direction == 3 && z_pos > 1){z_new_pos = z_pos - 1;}            //Bas
45.         if(direction == 4 && x_pos > 1){x_new_pos = x_pos - 1;}            //Gauche
46.      
47.         if(terrain[x_new_pos,z_new_pos] == - 6){
48.             terrain[x_new_pos,z_new_pos] = terrain[x_pos,z_pos] + high;
49.             x_pos = x_new_pos;
50.             z_pos = z_new_pos;
51.             Temp = Temp + 1;
52.         }else{
53.             x_pos = Random.Range(0 , X_dim);
54.             z_pos = Random.Range(0 , Z_dim);
55.         }
56.      
57.     }
58.  
59. //Creation
60.  
61.     for (x = 0; x < X_dim ; x++) {
62.         for (z = 0; z < Z_dim; z++) {
63.             var cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
64.             cube.transform.position = Vector3 (x, terrain[x,z], z);
65.         }
66.     }
67. }

Thank you !

 

The error appear on line 47