if (this.readyState == 4 && this.status == 200) {
Should be:
if (this.readyState == 4 && this.status == 200 && this.responseText != "") {
Then it won't do anything if the response is blank.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.