setTO method is not working with error message NullpointerException

import javafx.application.Application; import javafx.stage.Stage; import javafx.scene.Scene; import javafx.scene.control.Button; import javafx.scene.control.Label; import javafx.scene.layout.GridPane; import javafx.scene.layout.BorderPane; import javafx.concurrent.Task; import javafx.event.EventHandler; import javafx.event.ActionEvent;

public class TootAndOtto extends Application implements EventHandler{ private static TootAndOtto theObject; private int row = 6; private int col = 6; private Button[][] button = new Button[col][row]; private boolean nextT = false; private boolean clickMade = true; private boolean won = true; private boolean filled = false;

@Override public void start(Stage primaryStage) { theObject = this; GridPane gridPane = new GridPane(); button = new Button[col][row]; for(int i = 0; i

} public void handle(ActionEvent e) { Button b = (Button)e.getSource(); int c = findCol(b); int r = findRow(b); setTO(b,c); System.out.println(c); System.out.println(r);

} public int findCol(Button b) { int c = GridPane.getColumnIndex(b); return c; } public int findRow(Button b) { int r = GridPane.getRowIndex(b); return r; } public void setTO(Button b,int c) { for(int r = row -1; r >= 0; r--) { if(button[c][r].getText().isEmpty()){ button[c][r].setText("x"); } } } public void testButton(Button b) { button[2][3].setText("2,3"); button[1][0].setText("1,0"); button[0][1].setText("0,1"); } public static void main(String[] args) { Application.launch(args); }

}