JavaFx How to add buttons to GridPane with 2d array?

Question

I'm trying to make chess game with java only using Board class as GUI, Game class to run and test game rules, abstract Pieces as a parent class for each chess piece, Player class to indicate who's turn. I populated a board with 8 x 8 = 64 buttons first. To implement each piece's valid movement, I need to get each button's address or an index. Initially I didn't use 2d array but I thought it'd be easier to put the buttons in 2d array and get their index to control the movement of the pieces. This is what I tried so far and I don't know why I'm getting a NullPointer error.

I would appriciate any help or advice.

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;

public class Board extends Application {
    GridPane board = new GridPane();
    Scene s = new Scene(board);
    Button buttonArray[][];
    private void showBoard() {
        for (int i = 0; i < 8; i++) {
            for (int j = 0; j < 8; j++) {
                buttonArray[i][j] = new Button();


                buttonArray[i][j].setMinSize(70, 70);



                if ((i + j) % 2 == 0) { 
                    buttonArray[i][j].setStyle("-fx-background-color: #0e0f0f");
                } else {
                    buttonArray[i][j].setStyle("-fx-background-color: #ededed");
                }
                buttonArray[i][j].setOnAction(this::onSelect);

                board.add(buttonArray[i][j], i, j);
            }
        }
    }


    // event listener
    private void onSelect(ActionEvent event) {
        Button b = (Button) event.getSource();
        System.out.println("get source" + b);
        b.setStyle("-fx-background-color: magenta");

    }

    @Override
    public void start(Stage stage) throws Exception {
        showBoard();
        stage.setScene(s);
        stage.show();
    }

    public static void main(String args[]) {
        launch(args);
    }

}

//Error stackTrace

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at Board.showBoard(Board.java:15)
    at Board.start(Board.java:45)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
    ... 1 more
Exception running application Board