RemoveChild and go next frame don't work

I made a game that is based on collecting stars. I have a problem that is at the moment of meeting 4 stars want to share passed to the next frame and removed all the frames tried funkcjii removeChild but message pops up.

"TypeError: Error #1009: Cannot access a property or method of a null object reference."

 var health=20; 
var score=0;
health_txt.text=health.toString();
score_txt.text=score.toString();
var intervalPunkty = setInterval(addGwiazda,1000);


function addGwiazda(){
  var gwiazda:Gwiazda = new Gwiazda();
  gwiazda.x=Math.ceil(Math.random() * 550);
  gwiazda.y = -50;
  addChild(gwiazda);
  gwiazda.addEventListener(Event.ENTER_FRAME, dropGwiazda);




function dropGwiazda(e:Event){
  var b:Gwiazda = Gwiazda(e.target);
  b.y +=10;
  if(b.y > 400){
  b.removeEventListener(Event.ENTER_FRAME, dropGwiazda);
  removeChild(b);
  }






  if(jazda.hitTestObject(b)){
score ++;
  score_txt.text=score.toString();
  b.removeEventListener(Event.ENTER_FRAME, dropGwiazda);
  removeChild(b);



  if (score == 4){
gotoAndStop(15);


{
    this.parent.removeChildAt(0);
}
}


  }}}
  stop();