actionscript 3 - calling movieclip of a class from another class, error#1009 -

i got error: method of null object reference. i'm confused , don't know real cause.
got player movieclip on stage has instance of "player_mc", pass thru document class , player class.
player.as

import flash.display.*; import flash.events.*;   public class player extends movieclip {     public var myplayer:movieclip;      public function player(player:movieclip)     {         myplayer = player;          addeventlistener(event.enter_frame,on_enter);     } 

document.as

import flash.display.*; import components.player.player;  public class game_main extends movieclip {      public var player:player;       public function game_main()     {         player = new player(player_mc);       }  } 

now here think problem comes from. have green_enemy movieclip on stage has base class enemy.
enemy.as

import flash.display.movieclip; import components.player.player; import flash.events.event;  public class enemy extends movieclip {     var theplayer:player;       public function enemy()     {          this.addeventlistener(event.enter_frame,on_enter);     }      public function on_enter(e:event):void     {         if (this.hittestobject(theplayer.myplayer)) //calls player_mc player class         {             trace("hi");         }       }    } 

what on enemy function when enemy collides "player_mc" (which on stage) something. maybe code wrong.
help/tips appreciated. thanks!

in enemy.as see

var theplayer:player; 

not initialized. private, can not define outside. means exception comes here

this.hittestobject(theplayer.myplayer) 

you trying call myplayer null.

try define variable while constructing enemy class.

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to prevent null exception can check if theplayer null

public function on_enter(e:event):void {     if (theplayer && this.hittestobject(theplayer.myplayer))      {         trace("hi");     } }