Simple error but dunno what the problem is

1

anyone!!help me!!
this is driving me insane!!!

i got this error
Cannot access a property or method of a null object reference.

whenever I clicked on my button

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at test_fla::MainTimeline/onClick()

I know it means that the onClick is not on the stage,but I do put my buttons all in the same frame with the actionscript

here is my full actionscript,simple switch case

stop();

play_btn.addEventListener(MouseEvent.CLICK, onClick);
instruction_btn.addEventListener(MouseEvent.CLICK, onClick);
option_btn.addEventListener(MouseEvent.CLICK, onClick);
score_btn.addEventListener(MouseEvent.CLICK, onClick);
quit_btn.addEventListener(MouseEvent.CLICK, onClick);

function onClick(evt:MouseEvent):void {
    switch (evt.target) {
        case play_btn :
        trace("play");
            content_mc.gotoAndPlay("play");
            break;
        case instruction_btn :
            content_mc.gotoAndPlay("instruction");
            break;
        case option_btn :
            content_mc.gotoAndPlay("option");
            break;
        case score_btn :
            content_mc.gotoAndPlay("score");
            break;
        case quit_btn :
            fscommand("quit");
            break;
    }
}

the trace can works but not the link
i’m desperate :’(