请大神看看我这个哪里有错误，prompt之后出不来网页
<!DOCTYPE html>
<html>
<head>
<title> new document </title>
<meta http-equiv="Content-Type" content="text/html; charset=gbk"/>
<script type="text/javascript">
function openWindow() {
var a=confirm("是否打开新网页");
var b;
if(a){
b=prompt("确定打开的网址","https://www.baidu.com/");
if(b!=NULL){
window.open(b,'_blank','width =400px height=500px menubar=no toolbar=no');
}
}
}
</script>
</head>
<body>
<input type="button" value="新窗口打开网站" onclick="openWindow()" />
</body>
</html>
