fix this error:

"Exception in thread "main" java.lang.NullPointerException: Cannot read the array length because "tictactoegame.TicTacToeGame.gameboard" is null

at TicTacToeGame/tictactoegame.TicTacToeGame.displayBoard(TicTacToeGame.java:18)

at TicTacToeGame/tictactoegame.TicTacToeGame.main(TicTacToeGame.java:49)"

package tictactoegame;

public class TicTacToeGame {

static int[][] gameboard;

static final int EMPTY = 0;

static final int NOUGHT = -1;

static final int CROSS = 1;

/* Set a square on the board must be empty */

public static void set(int val, int row, int col) throws IllegalArgumentException {

if (gameboard[row][col] == EMPTY)

gameboard[row][col] = val;

else throw new IllegalArgumentException("Player already there!");

}

/* display the board */

public static void displayBoard() {

for(int r=0; r<gameboard.length; r++) {

System.out.print("|");

for (int c=0; c<gameboard[r].length; c++) {

switch(gameboard[r][c]) {

case NOUGHT:

System.out.print("O wins" );

break;

case CROSS:

System.out.print("Xwins" );

break;

default: // EMPTY

System.out.println("");

}

System.out.print("|");

}

System.out.println();

}}

public static void createBoard(int rows, int cols) {

//TODO Initialize the gameboard

}

public static void winOrTie() {

//TODO Determine whether X or O won or there is a tie

}

public static void main(String[] args){

createBoard(3,3);

int turn = 0;

int playerVal;

int outcome;

try(java.util.Scanner Scan= new java.util.Scanner(System.in)){;

do {

displayBoard();

playerVal = (turn % 2 == 0)? NOUGHT : CROSS;

if (playerVal == NOUGHT)

System.out.println (" O's turn");

else System.out.println(" X's turn" );

System.out.print("Enter row and column:" );

try {

set(playerVal, Scan.nextInt(), Scan.nextInt());

} catch (Exception ex)

{System.err.println(ex); turn--;}

turn ++;

outcome = NOUGHT;

}

while ( outcome == -2 );

displayBoard();

switch (outcome) {

case NOUGHT:

System.out.println("O wins!");

break;

case CROSS:

System.out.println("X wins!");

break;

case 0:

System.out.println("Tie.");

break;

}

}}}