nullpointerexception

public class NBullet extends Bullet
{
    public NBullet()
    {
        speed= 50;
       
        
        
    }
    /**
     * Act - do whatever the NBullet wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        shootingspeed();
        checkBullet();
        
    }    
     public void checkBullet()
    {
       Weapon w = new Weapon();
      
       Zombie z = new Zombie();
        
        
       
        Actor zombie = this.getOneIntersectingObject(Zombie.class);
        if(zombie != null )
        
        {
           
           getWorld().removeObject(this);
            
           
          
           getWorld().removeObject(zombie);
            
          
           
        }
         else if ( getX() <= 0|| getY()==0||getY() == 774)
        {  
              getWorld().removeObject(this);
        }
        
      
        
        
    }
   
    
}