err_file_not_found error in loading a page in extension

I am trying to load the same link of the tab into the popup window (this is not my primary purpose and i am doing this just to get acquainted) I am getting the error

Failed to load resource: net::ERR_FILE_NOT_FOUND

My js file is as follows

var pageGenerator = {
    requestPage: function() {
        var urlTosearch;
        chrome.windows.getCurrent(function(w) {
            chrome.tabs.getSelected(w.id,
            function (response){
                urlTosearch = response.url;
            });
        });
        var req = new XMLHttpRequest();
        req.open("GET", urlTosearch, true);
        req.onload = this.loadPage_.bind(this);
        req.send(null);
    },
    loadPage_: function (e) {
        var resp = e.target.responseText;
        document.body.insertAdjacentHTML( 'beforeend', resp);
    }
};
document.addEventListener('DOMContentLoaded', function () {
   pageGenerator.requestPage();
});

I have read that external pages can not be loaded onto the popup. Is it so? If true why? and if not how can it be done?