Hi, Error #1009

Hi, I have an animation with 4 buttons leading to 4 different frames  labelled a,b,c and d.

My buttons are named a_btn, b_btn,  c_btn,  d_btn

Of recent the first button is always giving me a a Error #1009 though it has been working for the last last year in the same animation.

the rest of the buttons work with no problem even though the code is same- help

Usha

Error message:

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

                at hdbcThrombophilia_fla::_5_extrinsicpathway_birdseye_16/frame1()

                at flash.display::MovieClip/gotoAndStop()

                at hdbcThrombophilia_fla::_5_extrinsicpathway_birdseye_16/fl_ClickToGoToAndStopAtFrame_a()

my code is

a_btn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame_a);

function fl_ClickToGoToAndStopAtFrame_a(event:MouseEvent):void
{
gotoAndStop("a");
}

b_btn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame_b);

function fl_ClickToGoToAndStopAtFrame_b(event:MouseEvent):void
{
gotoAndStop("b");
}

c_btn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame_c);

function fl_ClickToGoToAndStopAtFrame_c(event:MouseEvent):void
{
gotoAndStop("c");
}

d_btn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame_d);

function fl_ClickToGoToAndStopAtFrame_d(event:MouseEvent):void
{
gotoAndStop("d");
}