Objects on the stage are not read when jumping to their frame - actionscript-3

Question

I am working on an Air desktop application. At some point when the user presses a button, it will jump to a specific frame, the problem is after going to that frame some Movieclips on the stage at that frame are not read though they were read from the very beginning.
The following error occurs.
Error #1009: Cannot access a property or method of a null object
reference.
I do not know why he can not read what is already on the stage, I guess it is something related to the arrangement of the layers. I noticed that sometimes though I do not find it logical, It is supposed to read ALL that is in the same frame as long as it is stopped on it, right?
The project is as follows:
1) At the very beginning an intro is played and then it goes to the first frame of the course.
2) At first frame the user chooses one of 5 buttons to click, based on each one it goes to different frame.
3) When the user is at any frame it should return to the main frame if he clicked a back button, this button command is gotoAndStop(1) and some conditional removeChild() to clean the stage from any objects generated by code depending on the frame the function was called from.
4) The problem arises when this back button is clicked, one or more of the very first 5 buttons suddenly disappears and an error generated as - for some unknown reason - it can not be read by the program any more, it can not read any events for it and generates the above error.
My code is as follows:
var myLettersLoader:URLLoader= new URLLoader();
mainMenu.addEventListener(MouseEvent.CLICK,gotomainMenu);
letters.addEventListener(MouseEvent.CLICK,showLetters);
lessons.addEventListener(MouseEvent.CLICK,showLessons);
revision.addEventListener(MouseEvent.CLICK,showRevision);
myLettersLoader.load(new URLRequest("data/letters/letters.xml"));
myLettersLoader.addEventListener(Event.COMPLETE,loadXML);
function showLetters(e:MouseEvent)
{
//gotoAndStop(2)
//aaaaa.alpha=1;
//aaaaa.visible=true;
Tweener.addTween(e.currentTarget, {width:originalWidth,height:originalHeight, time:0.25, transition:"linear"});
myPlace.visible = true;
myPlace2.visible = false;
myPlace3.visible = false;
jewels.visible = false;
mainContainer.visible=false;
close.visible=false;
studentBook.visible = false;
mainButton = e.currentTarget.name;
Tweener.addTween(myPlace, {alpha:1, transition:"linear"});
lettersContainer.visible=true;
Tweener.addTween(letterContainerText, {alpha:1, transition:"linear"});
for (var i=1; i<29; i++)
{
var letter = "L" + i;
myPlace[letter].id = i;
myPlace[letter].alpha = 1;
myPlace[letter].addEventListener(MouseEvent.CLICK,gotoLetterFrame);
myPlace[letter].buttonMode = true;
}
}
function showLessons(e:MouseEvent)
{
Tweener.addTween(e.currentTarget, {width:originalWidth,height:originalHeight, time:0.25, transition:"linear"});
myPlace.visible= false;
myPlace2.visible = true;
myPlace3.visible = false;
jewels.visible = false;
mainContainer.visible=false;
close.visible=false;
studentBook.visible = false;
lettersContainer.visible=true;
mainButton = e.currentTarget.name;
Tweener.addTween(myPlace2, {alpha:1, transition:"linear"});
studentBook.alpha = 0;
for (var i=0; i<5; i++)
{
var lesson = "Lesson" + i;
myPlace2[lesson].id = i;
myPlace2[lesson].alpha = 1;
myPlace2[lesson].addEventListener(MouseEvent.CLICK,gotolessonFrame);
myPlace2[lesson].buttonMode = true;
}
}
//=======================Revision functions==================================
function showRevision(e:MouseEvent)
{
Tweener.addTween(e.currentTarget, {width:originalWidth,height:originalHeight, time:0.25, transition:"linear"});
myPlace.visible= false;
myPlace2.visible = false;
myPlace3.visible = true;
jewels.visible = false;
mainContainer.visible=false;
close.visible=false;
studentBook.visible = false;
lettersContainer.visible=true;
mainButton = e.currentTarget.name;
Tweener.addTween(myPlace3, {alpha:1, transition:"linear"});
studentBook.alpha = 0;
for (var i=0; i<7; i++)
{
var revision = "Revision" + i;
myPlace3[revision].id = i;
myPlace3[revision].alpha = 1;
myPlace3[revision].addEventListener(MouseEvent.CLICK,gotoRevisionFrame);
myPlace3[revision].buttonMode = true;
}
}
//========================================================
function gotoLetterFrame(e:MouseEvent)
{
reloadButton.visible=true;
mainMenu.visible=true;
myClose.visible=true;
reloadButton.visible=true;
myNext.visible=true;
currentTarget=(e.currentTarget.id-1);
currentName = arrOfLetters[currentTarget];
xmlListOfClass=new XMLList(myxml.letter.(#id==currentName).children());
gotoAndStop(xmlListOfClass[counter].localName());
abc.visible=abcd.visible=true;
mainMenu.buttonMode=true;
}
function gotolessonFrame(e:MouseEvent)
{
reloadButton.visible=true;
mainMenu.visible=true;
myClose.visible=true;
reloadButton.visible=true;
myNext.visible=true;
currentTarget=(e.currentTarget.id);
xmlListOfClass = new XMLList(lessonsArr[currentTarget].lesson.children());
gotoAndStop(xmlListOfClass[counter].localName());
abc.visible=abcd.visible=true;
mainMenu.buttonMode=true;
}
function gotoRevisionFrame(e:MouseEvent)
{
reloadButton.visible=true;
mainMenu.visible=true;
myClose.visible=true;
reloadButton.visible=true;
myNext.visible=true;
currentTarget=(e.currentTarget.id);
myRevisionLoader.load(new URLRequest("data/revisions/"+currentTarget+"/revision.xml"));
myRevisionLoader.addEventListener(Event.COMPLETE,loadRevisionXML);
}
//=====================================
function loadLessonXML(e:Event)
{
lessonsArr[xx] = new XML(e.target.data);
xx++;
}
//==============================For revision==================================
function loadRevisionXML(e:Event)
{
revisionArr = new XML(e.target.data);
xmlListOfClass = new XMLList(revisionArr.revision.children());
gotoAndStop(xmlListOfClass[counter].localName());
abc.visible=abcd.visible=true;
mainMenu.buttonMode=true;
}
function loadXML(e:Event)
{
myxml = new XML(e.target.data);
}
//====================================
function gotomainMenu(e:MouseEvent)
{
gotoAndPlay(1);
}
This code is in the first frame, and in the second frame the button mainButton
is the button responsible for going back to frame 1
The buttons lessons,letters,revision disappears when returning to frame 1, or one of them sometimes with no logical reason

Answer 1

I solved it.It was all about the arrangement of layers.
All what I did is that I put the layer which contains the 5 buttons under the other layers. This surprisingly solved the problem. I do not know the reason till this moment but all what I know is that for some reason the caller of some event must be under the other layers containing some other objects or at least at the same layer.