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

I need a little help. I'm making a game, and everything works fine except for the fact that I have 2 errors, one that doesn't bother me as I can still use the button, but the other one won't let me use the button.

Error:

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

    at Untitled_2_fla::MainTimeline/frame2()[Untitled_2_fla.MainTimeline::frame2:207]

    at flash.display::MovieClip/gotoAndStop()

    at Untitled_2_fla::MainTimeline/PlayBtn()[Untitled_2_fla.MainTimeline::frame1:13]

//Magyx Placement

RedAtk.addEventListener(MouseEvent.CLICK, RedAttack);

function RedAttack(Event:MouseEvent):void
{
    gotoAndStop(19)
}

That's where Frane2:207 is, I don't see anything wrong with it, considering all my other ones are the same

nextBlueAtk.addEventListener(MouseEvent.CLICK, NWA);

function NWA(Event:MouseEvent):void
{
    gotoAndStop(23)
}

nextGreyAtk.addEventListener(MouseEvent.CLICK, NGA);

function NGA(Event:MouseEvent):void
{
    gotoAndStop(27)
}

nextGreenAtk.addEventListener(MouseEvent.CLICK, NGRA);

function NGRA(Event:MouseEvent):void
{
    gotoAndStop(31)
}

I get no errors for those ones.

And frame1:13?

function PlayBtn(event:MouseEvent):void
{
    gotoAndStop(2)
}

Someone please help, I want to continue working on this game!