title: HaxeFlixel - FlxSubState unexpected null object reference
body: <p>Im trying to make simple ingame menu. Im using FlxSubState class. First im trying invoke substate in main game state with <code>openSubState(gameMenu);</code> after pressing Esc.</p>
  
  <p>There is code in my substate class, this class inheriting <code>FlxSubState</code>: </p>
  
  <pre><code>override public function create():Void 
  {
      super.create();     
      continueButton = new FlxButton(0,0, "Continue", continueGame);
      continueButton.x = FlxG.width / 2 - continueButton.width / 2;
      continueButton.y = FlxG.height / 2 - continueButton.height / 2;
      add(continueButton);
  }
  
  private function continueGame():Void
  {
      close();    
  }
  </code></pre>
  
  <p>Problem is : everytime after click on <code>continueButton</code> game crash with null exception in <code>FlxTypedGroup</code>. I think its in <code>close();</code> method but i really cannot figure it out. Can anyone help me ? 
  Or suggest better way to implement ingame menu ? </p>