window.open 传值为空？

Question

父页面通过window.open打开的子页面father.html<input type="" name="main" id="mmm" value="123" /><script type="text/javascript">     $('#Btn').on('click',function(){        window.open('son.html');     });</script>son.html    <script type="text/javascript">                  alert(window.opener.document.getElementById('mmm').value);    </script>控制台Uncaught TypeError: Cannot read property 'value' of null at son.html:21