ActionScript 3 - cannot move to the next scene.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at CompangleComplete_fla::MainTimeline/loop()[CompangleComplete_fla.MainTimeline::frame119:205]

I got this error.

When the player touch the "black rectangle", supposed to move to the next scene.

I try to check where is the problem, then I try this

if(endpoint==false){
if(player.hitTestObject(back.other)){

endpoint==true;
trace("next scene");}

the code run without error.

But then I try this,

if(endpoint==false){
if(player.hitTestObject(back.other)){

endpoint==true;
gotoAndPlay(2, "Scene 9");}

I got error like above.

The code for my gotoAndPlay(2, "Scene 9") is wrong. how i should change it?