JavaScript TypeError: Cannot read property 'style' of null

In your script, this part:

document.getElementById('Noite')

must be returning null and you are also attempting to set the display property to an invalid value. There are a couple of possible reasons for this first part to be null.

1. You are running the script too early before the document has been loaded and thus the Noite item can't be found.

2. There is no Noite item in your HTML.

I should point out that your use of document.write() in this case code probably signifies a problem. If the document has already loaded, then a new document.write() will clear the old content and start a new fresh document so no Noite item would be found.

If your document has not yet been loaded and thus you're doing document.write() inline to add HTML inline to the current document, then your document has not yet been fully loaded so that's probably why it can't find the Noite item.

The solution is probably to put this part of your script at the very end of your document so everything before it has already been loaded. So move this to the end of your body:

document.getElementById('Noite').style.display='block';

And, make sure that there are no document.write() statements in javascript after the document has been loaded (because they will clear the previous document and start a new one).

In addition, setting the display property to "display" doesn't make sense to me. The valid options for that are "block", "inline", "none", "table", etc... I'm not aware of any option named "display" for that style property. See here for valid options for teh display property.

You can see the fixed code work here in this demo: http://jsfiddle.net/jfriend00/yVJY4/. That jsFiddle is configured to have the javascript placed at the end of the document body so it runs after the document has been loaded.

P.S. I should point out that your lack of braces for your if statements and your inclusion of multiple statements on the same line makes your code very misleading and unclear.

I'm having a really hard time figuring out what you're asking, but here's a cleaned up version of your code that works which you can also see working here: http://jsfiddle.net/jfriend00/QCxwr/. Here's a list of the changes I made:

1. The script is located in the body, but after the content that it is referencing.
2. I've added var declarations to your variables (a good habit to always use).
3. The if statement was changed into an if/else which is a lot more efficient and more self-documenting as to what you're doing.
4. I've added braces for every if statement so it absolutely clear which statements are part of the if/else and which are not.
5. I've properly closed the </dd> tag you were inserting.
6. I've changed style.display = ''; to style.display = 'block';.
7. I've added semicolons at the end of every statement (another good habit to follow).

The code:

<div id="Night" style="display: none;">
    <img src="Img/night.png" style="position: fixed; top: 0px; left: 5%; height: auto; width: 100%; z-index: -2147483640;">
    <img src="Img/moon.gif" style="position: fixed; top: 0px; left: 5%; height: 100%; width: auto; z-index: -2147483639;">
</div>    
<script>
document.write("<dl><dd>");
var day = new Date();
var hr = day.getHours();
if (hr == 0) {
    document.write("Meia-noite!<br>Já é amanhã!");
} else if (hr <=5 ) {
    document.write("&nbsp;&nbsp;Você não<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;devia<br>&nbsp;&nbsp;&nbsp;&nbsp;estar<br>dormindo?");
} else if (hr <= 11) {         
    document.write("Bom dia!");
} else if (hr == 12) {
    document.write("&nbsp;&nbsp;&nbsp;&nbsp;Vamos<br>&nbsp;almoçar?");
} else if (hr <= 17) {
    document.write("Boa Tarde");
} else if (hr <= 19) {
    document.write("&nbsp;Bom final<br>&nbsp;de tarde!");
} else if (hr == 20) {
    document.write("&nbsp;Boa Noite"); 
    document.getElementById('Noite').style.display='block';
} else if (hr == 21) {
    document.write("&nbsp;Boa Noite"); 
    document.getElementById('Noite').style.display='none';
} else if (hr == 22) {
    document.write("&nbsp;Boa Noite");
} else if (hr == 23) {
    document.write("Ó Meu! Já é quase meia-noite!");
}
document.write("</dl></dd>");
</script>

This happens because document.write would overwrite your existing code therefore place your div before your javascript code. e.g.:

CSS:

#mydiv { 
  visibility:hidden;
 }

Inside your html file

<div id="mydiv">
   <p>Hello world</p>
</div>
<script type="text/javascript">
   document.getElementById('mydiv').style.visibility='visible';
</script>

Hope this was helpful