Action Script 3 code not working for start and stop button?

Question

Ok so I have this simple animation I created of a circle that moves from one side of the stage to the other. I have added a new layer and called it buttons. On this layer I have added 2 buttons. One for start and another one for stop. The purpose is to get my circle to move from one side of the stage to the other but be able to use my buttons so that I can start and stop the animations at random times during playback. I fixed all my compiler errors now the problem lies in that everytime I click the start or the stop button I get an output error. I have a 3rd layer in which is titled actions and this is where all my code is posted. I removed that layer and placed my code in the first frame of the buttons layer to see if this would change anything but I still get the same output errors. So I just added back my actions layer. What could I be doing wrong? I have made sure to name all my movie clips and buttons correctly and I even added an instance name to them.
Here is my code and the errors I am getting when I press the play and stop button on test-
start_btn.addEventListener(MouseEvent.CLICK, startCircle);
stop_btn.addEventListener(MouseEvent.CLICK, stopCircle);
function startCircle(e:MouseEvent):void{
    circle.play();
function stopCircle(e:MouseEvent):void{
    circle.stop();
green_btn.addEventListener(MouseEvent.CLICK, greenCircle);
red_btn.addEventListener(MouseEvent.CLICK, redCircle);
function greenCircle(e:MouseEvent):void{
    circle.play();
function redCircle(e:MouseEvent):void{
    circle.stop();
Here are my output errors-
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at exercise2_fla::MainTimeline/redCircle()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at exercise2_fla::MainTimeline/greenCircle()

Answer 1

ok so my circle is my movie clip and it is titled circle and the instance name is circle. Am I supposed to change MovieClip to circle? I put the code at the top of my actions script but for some reason it did not work. I did change play to stop and it got the movie clip to stop when I do test movie but my buttons still do not work. So I changed stop back to play. Strange.
I did try changing MovieClip to circle but that did not work so I did Circle with a capital C and that did me no good eigther.
I meant to add that I did get an error that said- 1180 Call to a possibly undefined method circle.