Getting error when click

programming flash
1

hii, i’m developing one small interactive game and i’m getting this error, when i’m trying to click on button.
error #1009: Cannot access a property or method of a null object reference.
at Geese_And_Tortoise_PG01_Animation_V02_fla::Geese01 _Null_Animation_5/onMouseClick()

my action code for that is:

import flash.events.MouseEvent;

act_btn.addEventListener(MouseEvent.CLICK,onMouseC lick);
act_btn.addEventListener(MouseEvent.CLICK,onOut);

function onMouseClick(event:MouseEvent):void
{

 quick01.visible=false; (quick01 is movieclip1 instance name)
 quack01.visible=true;(quack01 is moviclip2 instance name)
 quack01.play();
 }
 
 function onOut(event:MouseEvent):void 
 { 

   
   quack01.visible=false;
   quick01.visible=true;
   

 }

why i’m getting this error, can anybody plz tel me