How to set focus on linb.UI.Dialog

Hello to all

First, sorry for my poor english, but I have one problem:

I create a login dialog whit ajax function. When user try to log and enter wrong password and/or username, the script popup linb.UI.Dialog.alert. When user click on OK button, the script close linb.UI.Dialog.alert, but I can't return focus on login dialog!

Please help me!

code:

_button6_onclick:function (profile, e, src, value) {
			//alert(this.username.getUIValue());
            var mc=this;
			var post = new Object();
			post.username=this.username.getUIValue();
            post.username_value=this.username.getDomId();
			post.password=this.password.getUIValue();
            post.password_value=this.password.getDomId();
            post.dialog=this.dialog1.getDomId();
            
            
            linb.Ajax( "login.html", post, function(rep){
               var obj = _.unserialize(rep);
			   if (obj.error!="0")
			   {
				    linb.UI.Dialog.alert('Грешка',obj.error,function(){ 
						mc.username.setUIValue('');
                        alert('close');
                        linb('username_input').focus();
                        //linb('linb.UI.Input-INPUT:a:').focus();
                        //document.getElementById("linb.UI.Input-INPUT:a:").focus(); 
                        //mc.username.focus();
					})
			   } else {
					window.location.replace("http://<?$_SERVER["SERVER_NAME"]?>/index.html");
			   }
            }).start();
        }