Create A Matrix Of JTextFields?

I am making a Sudoku game and creating a matrix of JTextFields. However I am getting the following errors

Exception in thread "main" java.lang.NullPointerException
at SudokuView.board(SudokuView.java:30)
at SudokuView.<init>(SudokuView.java:18)
at SudokuMain.main(SudokuMain.java:5)

I know the problem is with this code

box[i][j] = new JTextField();
panel.add(box[i][j]);

I know this because when I do this:

panel.add(new JTextField());

It works. However it puzzles me why it is not working.

Whole Code:

import java.awt.GridLayout;
import javax.swing.*;
 public class SudokuView {
 JFrame frame;
JPanel panelBoard;
JTextField[][] box;
int row=10; int col=10;
 SudokuView(){
frame = new JFrame("Play Sudoku GOOD LUCK");

[code]....