Properly, you should always put form elements such as <textarea> into a <form>...</form>.
If you don't have a <form>, just wrap it with a dummy one:
Won't do any harm and avoids problems in older browsers.
But if you do *NOT* have it in a <form>, then it is better to leave it as is. Don't give it a name.
Instead, just change the JS code from
var obj = document.form.message;
var obj = document.getElementById("new_topic_text");
For that matter, you can use that code whether or not you put it in a <form>.