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

error:

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

          at test1_fla::MainTimeline/openPage1()[test1_fla.MainTimeline::frame2:82]

at test1_fla::MainTimeline/onClickBtn1()[test1_fla.MainTimeline::frame2:52]

code:

line 77 -83:

function openPage1():void

{

          var pageNum11:int;

          pageNum11 = pageNum1 - 1;

          trace("tween" + pageNum11 + pageNum1);

          mainMenu.tweenText.getChildByName("tween" + pageNum11 + pageNum1).addTarget(mainMenu.pane1.text1, 1);

}

line 46-59:

function onClickBtn1(e:MouseEvent):void

{

          switch(e.target)

          {

                    case mainMenu.pane1.btnDown:

                              pageNum1 += 1;

                              openPage1();

                              break;

                    case mainMenu.pane1.btnUp:

                              openPage1();

                              pageNum1 -= 1;

                              break;

          }

}

Can anyone tell me where did I do wrong?