Problem with instantiating prefab object

I don't know where the problem is: i wanted to instantiate a (random number)x(random number) platform using arrays and a prefab of a brick tile. Don't show any other solutions to achieve this goal, i just want a solution for this.

Error: NullReferenceException: Object reference not set to an instance of an object general_Script.posX () (at Assets/Scripts/general_Script.cs:49) general_Script.Start () (at Assets/Scripts/general_Script.cs:21)

 public int dunNum;
 public int dunNumSqrt;
 private float[,] dunPosx;
 private float[,] dunPosy;
 public int dunDim;
 public float tileDim = 0.16f;
 public GameObject go;


 void Start () {
     dunNum = (int)Mathf.Pow(Random.Range(1,5),2);
     dunNumSqrt = (int)Mathf.Sqrt(dunNum);

     float[,] dunPosx = new float[dunNumSqrt,dunNumSqrt];
     float[,] dunPosy = new float[dunNumSqrt,dunNumSqrt];

     posX();
     posY();
     instantiateTile();






     

}

 void Update () {}

 void posX()
 {

     for (int i = 0;i < dunNumSqrt;i++)
     {
         for (int j = 0;j < i;j++)
         {
             dunPosx[i,j] = 0.16f*j;
         }
     }
 }

 void posY()
 {

     for (int i = 0;i < dunNumSqrt;i++)
     {
         for (int j = 0;j < i;j++)
         {
             dunPosy[i,j] = 0.16f*i;
         }
     }

 }
 void instantiateTile()
 {
 for(int i = 1;i < dunNumSqrt;i++)
 {
     for(int j = 1;j < dunNumSqrt;j++)

     {    
       Instantiate(go,new Vector3(dunPosx[i,j],dunPosy[i,j],0),Quaternion.identity);

     }
     
 }

} }