Buttons to Navigate forward and Back but gets Stuck in Back.

Hey guys, I am making a project for my college class and it has to do with creating an interactive environment in flash CS4.  Similar to 99 rooms if anyone has gone and seen that famous piece of work.

Anyways I am using action script 3.0 and I created multiple layers using just one scene.  I created a layer with lables on them to mark where I want my buttons to point to and go to.  I have the stop (); on the first frame.  And I made my buttons mostly interactive with movie symbols and tweens.

My first scene I have 2 doors, the user can pick which door to go to and it takes them into a different path.

I made the buttons that are clicked to go forward, and then I tried to make an invisible button on the button edge of the scene to go backwards.  However, the PROBLEM is when I click my button to go backwards I am stuck on the first page and can't move forward.

My AS code is this

function gotoOne(evt:Event):void{
    gotoAndStop("one")
}
backbutton1.addEventListener("click",gotoOne);

function gotoFour(evt:Event):void{
    gotoAndStop("four")
}
buttonforward1.addEventListener("click",gotoFour);

Basically to sum it up the first code is of course for the button to go backwards.  And the forward buttons skips a scene to go forward.  I have another code for the 2nd door to go back as well.

function gotoOne1(evt:Event):void{
    gotoAndStop("one")
}
backbutton2.addEventListener("click",gotoOne1);

I made it say gotoOne1 because I have 2 rooms going back to the same home page room.  So it prevents stuff from going out of whack.

Now here are my error messages.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at finalprojectPractice1_fla::MainTimeline/frame1()
    at flash.display::MovieClip/gotoAndStop()
    at finalprojectPractice1_fla::MainTimeline/gotoOne()

So I am not sure what to do, even my Professor took an hour to try to help me and she couldnt figure it out.  And it would make my project 100 percent better if I can go backward and forward in my environment with my buttons.  Hope you guys can help