Why any action with Graphics crashing?

package hello;
 
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.Sprite;
import javax.microedition.lcdui.game.LayerManager;
import javax.microedition.lcdui.Graphics;
 
public class Tiles extends MIDlet {
 
  Board b;
 
  public Tiles() {
    b = new Board(this);
  }
 
  public void startApp() {
    Display.getDisplay(this).setCurrent(b);
  }
 
  public void pauseApp() {
  }
 
  public void destroyApp(boolean unconditional) {
  }
 
}
 
class Board extends Canvas implements CommandListener {
private GameDesign gameDesign;
private Sprite cursor;
private LayerManager lm;
 
public Graphics g;
public int x = 0;
  MIDlet midlet;
 
  Command exitCommand;
 
  public Board(MIDlet midlet_) {
      try{
          this.lm = new LayerManager();
 
this.gameDesign = new GameDesign();
this.cursor = this.gameDesign.getCurs();
this.cursor.defineReferencePixel(cursor.getWidth(), cursor.getHeight());
this.cursor.move(0, 0);
          this.lm.append(cursor);
          
          this.lm.setViewWindow(0, 0, this.getWidth(), this.getHeight());
      }
      catch(Exception x){}
 
    exitCommand = new Command("Exit", Command.SCREEN, 2);
    addCommand(exitCommand);
 
    setCommandListener(this);
 
    ////////////////////===============
 
 
    repaint();
 
  }
 
  public void commandAction(Command c, Displayable d) {
    if (c == exitCommand) {
      midlet.notifyDestroyed();
 
    }
  }
 
  public void paint(Graphics g) {
      g.setColor(255);
      g.fillRect(0, 0, 100, 100);
            //g.drawImage(im, x, 10, Graphics.LEFT | Graphics.TOP);
            this.cursor.paint(g);
  }
 
  public void keyPressed(int code) {
    int game = getGameAction(code);
 
    switch (game) {
    case Canvas.UP:
      System.out.println("Canvas.UP");
      break;
    case Canvas.DOWN:
      System.out.println("Canvas.DOWN");
      break;
    case Canvas.LEFT:
      System.out.println("Canvas.LEFT");
      break;
    case Canvas.RIGHT:
        x++;
        this.cursor.setPosition(x, cursor.getY());
        this.cursor.paint(g);      //<===============NULL POINTER EXCEPTION
      System.out.println("Canvas.RIGHT");
      break;
    }
 
  }
 
 
}