Javascript open.window returns null in called function

I have developed a function that, when called, should open a window, but return null

. If I open the window in a native JavaScript function, it works. I suppose its original function passes control to another function, but for some reason it doesn't work.

Here is my original function, this basically calls a new JavaScript file and loads the HTML file, and when "ready" it needs to display window.open with the HTML file now in string form.

order.prototype.printMe = function() {
    order_resume.loadthis("myTestPage.html", "showData");

    // OPENING WINDOW HERE WORKS; but the the html file that is loaded
    // in above line hasn't finsihed loading - so i need to show it form
    // the function below once in "ready" state 

/*      child1 = window.open ("about:blank","_blank");
        child1.document.write( myDocument );
        child1.document.close();
*/      
}

      

        
        
        
      

    

And here is my function called from the original function:

function showResume() {
    this.req = false;

    reservaResumen.prototype.showData = function() {
        if (this.req.readyState == 4) {
            child1 = window.open("about:blank", "_blank"); /// THIS RETURNS NULL
            child1.document.write("test");
            child1.document.close();
        }
    }

    reservaResumen.prototype.loadthis = function(url, myMethod) {
        if (window.XMLHttpRequest && !(window.ActiveXObject)) {
            try {
                this.req = new XMLHttpRequest();
            }
            catch (e) {
                this.req = false;
            }
        }
        else if (window.ActiveXObject) {
            try {
                this.req = new ActiveXObject("Msxml2.XMLHTTP");
            }
            catch (e) {
                try {
                    this.req = new ActiveXObject("Microsoft.XMLHTTP");
                }
                catch (e) {
                    this.req = false;
                }
            }
        }
        if (this.req) {
            var loader = this;
            this.req.onreadystatechange = function() {
                eval("loader." + myMethod + ".call(loader)")
            }
            this.req.open("GET", url, true);
            this.req.send("");
        }
    }​