Palette can't access active document

Hello !

I am working on a randomizer script for illustrator, and i am stubling on a problem.

I would like my script to be a palette, but when i change 

new Window("dialog", ...)

to 

new Window("palette", ...)

my script can no longer call app.activeDocument .

Here are the bits of code interesting :

#target illustrator
#targetengine "session"

var dialogUI = (function () {
	var palette = new Window("palette", undefined, undefined, {closeButton: true, resizeable: true}); 
	[...]
	var apply = group_button.add("button", undefined, undefined, {name: "apply"}); 
	apply.text = "apply"; 

	apply.addEventListener("click",function(){
		MyDoc = app.activeDocument;
		MySelection = MyDoc.selection;
		if(MySelection instanceof Array){
			if(tab_selectionnee == tab_fill_colour){
				random_background_color()
			}
		}
		app.redraw()
	})
	
	palette.show();
}());

The error is where MyDoc = app.activeDocument; is, and MySelection = ... is never called

Does anyone have an idea on how to solve this ?