You are creating Window object inside eventhandler and when function finishes garbage collector removes all local variables from it, including appCoreWindow.
Try this instead:
var win = Ti.UI.currentWindow;
var appCoreWindow = Ti.UI.createWindow({ url:"core.js" });
var btn = Ti.UI.createButton({.....});
btn.addEventListener('click',function(e){
appCoreWidnow.open();
win.close();
});
win.add(btn);