TypeError: document.getelementbyid(…) is null

<html>
<head>
<title>Port Scanner for Mozilla Firefox</title>
</head>
<body>
<form name="my_form" id="my_form" method="post" action="#" onsubmit="return displayMessage();">
<p align="center">URL/Domain Name/IP Address: <input type="text" size="20" name="uri" value="" placeholder="http://www.google.com" required/></p>
<table align="center" border="1" width=100% bgcolor="#f4fff0" bordercolor="green" cellspacing="2" cellpadding="2">…
TypeError: document.getelementbyid(…) is null

See original post by AbhijeetMuneshwar