Cannot access a property or method of a null object reference

1

Why is it that ActionScript Code:
[LEFT]getChildByNameCOLOR=#000000[/COLOR].[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]false[/COLOR];
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works and ActionScript Code:
[LEFT]getChildByNameCOLOR=#000000[/COLOR].[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]false[/COLOR];
[/LEFT]

give this error:

ActionScript Code:
[LEFT]TypeError: [COLOR=#0000FF]Error[/COLOR] [COLOR=#808080]*#1009: Cannot access a property or method of a null object reference.*[/COLOR]
at MethodInfo-[COLOR=#000080]98[/COLOR][COLOR=#000000]([/COLOR][COLOR=#000000])[/COLOR]

[/LEFT]

ActionScript Code:
[LEFT]getChildByName[COLOR=#000000]([/COLOR][COLOR=#FF0000]"Promotion1"[/COLOR][COLOR=#000000])[/COLOR].[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]**false**[/COLOR];

[/LEFT]

works well in the FLA timeline. But not in the class. It’s parent is the main timeline

Fla:
ActionScript Code:
[LEFT][COLOR=#0000FF]import[/COLOR] image.[COLOR=#000080]imageContainer[/COLOR];
[COLOR=#000000]var[/COLOR] promoHolder:[COLOR=#0000FF]Array[/COLOR] = [COLOR=#000000]new[/COLOR] [COLOR=#0000FF]Array[/COLOR]COLOR=#000000[/COLOR];
[COLOR=#0000FF]for[/COLOR] [COLOR=#000000]([/COLOR][COLOR=#000000]var[/COLOR] i:[COLOR=#0000FF]int[/COLOR] = [COLOR=#000080]1[/COLOR]; i < [COLOR=#000080]5[/COLOR]; i++[COLOR=#000000])[/COLOR] [COLOR=#000000]{[/COLOR]
[COLOR=#0000FF]this[/COLOR][COLOR=#000000][[/COLOR][COLOR=#FF0000]‘Promotion’[/COLOR]+i[COLOR=#000000]][/COLOR] = [COLOR=#000000]new[/COLOR] imageContainerCOLOR=#000000[/COLOR];
promoHolder[COLOR=#000000][[/COLOR]i[COLOR=#000000]][/COLOR] = [COLOR=#0000FF]this[/COLOR][COLOR=#000000][[/COLOR][COLOR=#FF0000]‘Promotion’[/COLOR]+i[COLOR=#000000]][/COLOR];
promoHolder.[COLOR=#0000FF]push[/COLOR]COLOR=#000000[/COLOR];
addChildCOLOR=#000000[/COLOR];
promoHolder[COLOR=#000000][[/COLOR]i[COLOR=#000000]][/COLOR].[COLOR=#0000FF]name[/COLOR] = [COLOR=#FF0000]“Promotion”[/COLOR] +i;

[COLOR=#000000]}[/COLOR]

[COLOR=#000000]function[/COLOR] testCOLOR=#000000[/COLOR]:[COLOR=#0000FF]void[/COLOR][COLOR=#000000]{[/COLOR]
[COLOR=#000000]}[/COLOR]

[COLOR=#000000]**var**[/COLOR] Promotion1:[COLOR=#0000FF]MovieClip[/COLOR] = [COLOR=#0000FF]MovieClip[/COLOR][COLOR=#000000]([/COLOR]getChildByName[COLOR=#000000]([/COLOR][COLOR=#FF0000]"Promotion1"[/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000])[/COLOR];
[COLOR=#000000]**var**[/COLOR] Promotion2:[COLOR=#0000FF]MovieClip[/COLOR] = [COLOR=#0000FF]MovieClip[/COLOR][COLOR=#000000]([/COLOR]getChildByName[COLOR=#000000]([/COLOR][COLOR=#FF0000]"Promotion2"[/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000])[/COLOR];
[COLOR=#000000]**var**[/COLOR] Promotion3:[COLOR=#0000FF]MovieClip[/COLOR] = [COLOR=#0000FF]MovieClip[/COLOR][COLOR=#000000]([/COLOR]getChildByName[COLOR=#000000]([/COLOR][COLOR=#FF0000]"Promotion3"[/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000])[/COLOR];
[COLOR=#000000]**var**[/COLOR] Promotion4:[COLOR=#0000FF]MovieClip[/COLOR] = [COLOR=#0000FF]MovieClip[/COLOR][COLOR=#000000]([/COLOR]getChildByName[COLOR=#000000]([/COLOR][COLOR=#FF0000]"Promotion4"[/COLOR][COLOR=#000000])[/COLOR][COLOR=#000000])[/COLOR];

[COLOR=#808080]*//Promotion1.visible=false;*[/COLOR]
Promotion2.[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]**false**[/COLOR];
Promotion3.[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]**false**[/COLOR];
Promotion4.[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]**false**[/COLOR];

[/LEFT]

Class:
ActionScript Code:
[LEFT]…
[COLOR=#000000]function[/COLOR] handleClickCOLOR=#000000[/COLOR]:[COLOR=#0000FF]void[/COLOR] [COLOR=#000000]{[/COLOR]
[COLOR=#0000FF]trace[/COLOR]COLOR=#000000[/COLOR];

getChildByNameCOLOR=#000000[/COLOR].[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]false[/COLOR];
getChildByNameCOLOR=#000000[/COLOR].[COLOR=#0000FF]visible[/COLOR]=[COLOR=#000000]false[/COLOR];
[COLOR=#000000]}[/COLOR]
…
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