Remove a child completely from the memory

I've got a problem that I can't figure out.

I've got in my Engine Class (engine.as) this :

public static var viseur:Viseur;  
var lookCocoDessous:Boolean = thisBack == "cocotierDessous"; 
if (lookCocoDessous) { 
viseur = new Viseur(stage); 
stage.addChild(viseur); 
viseur.visible = true; 
souris.visible = false; 
puzzle.addListeners();  }

And in my puzzle class (Puzzle.as) this :

public static var viseur:Viseur;

and when the function "backToJardin" is called I want to remove "viseur" (which has been called in the Engine.as). So I wrote this :

public function backToJardin(thisBack:String😞void{  
Engine.viseur.stage.removeChild(viseur); 
Engine.newBack = "jardin"; 
stageRef.dispatchEvent(new Event("changeBackground"));  }

But I've got this error :

TypeError: Error #2007: Parameter child must be non-null. at flash.display::DisplayObjectContainer/removeChild() at com.laserdragonuniversity.alpaca::Puzzle/backToJardin()

Do you know why ? And how can I resolve it ?

I'd like to remove COMPLETELY "viseur" when the function is called in my Puzzle.as (it will never be called again, so I want to "destroy" it)

I've tried to put

Engine.removeViseur();

in my backToJardin function

and put

public function removeViseur(){

stage.removeChild(viseur);}

in my Engine class, but I've got this error : error 1061 removeViseur can not be defined through a reference with static type

(and same thing if I put that in my Engine.as class) :

public function removeViseur(){ 
if (Engine.viseur && Engine.viseur.parent) { 
Engine.viseur.parent.removeChild(viseur); } 
}