Qooxdoo Remote Request Failed (undefined)

var lbl_usr = new qx.ui.basic.Label("Benutzername");  
var tf_usr = new qx.ui.form.TextField();
var lbl_pass = new qx.ui.basic.Label("Passwort");  
var pf_pass = new qx.ui.form.PasswordField();	
var btn_login = new qx.ui.form.Button("->...");  

var doc = this.getRoot();

doc.add(lbl_usr,   { left: 100 ,top: 50  } );
doc.add(tf_usr,    { left: 190 ,top: 50  } );	
doc.add(lbl_pass,  { left: 100 ,top: 80  } );
doc.add(pf_pass,   { left: 190 ,top: 80  } );
doc.add(btn_login, { left: 170 ,top: 110 } );

btn_login.addListener("execute", function(e) {  
      	if( !tf_usr.getValue() || !pf_pass.getValue() ) {
      		alert("Bitte vollständig füllen");  
        }
        else {
        	
        	var req = new qx.io.remote.Request("http://localhost/index.php", "GET", "text/plain");  
        	
        	req.addListener("failed", function(e,f) {  
        		//alert(e.getContent());
        		alert(e + " " + f );  
        	});	
        	req.addListener("completed", function(e) {  
        		alert(e.getContent());
        	});
        	req.send();
        	alert("Else ende...");  
        }
      
	});
    }
  }
});

<?php

echo "hallo"  

?>