<pre><code>&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
&lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;meta http-equiv="X-UA-Compatible" content="IE=edge"&gt;
    &lt;meta name="viewport" content="width=device-width, initial-scale=1.0"&gt;
    &lt;title&gt;Color Flipper&lt;/title&gt;
    &lt;script src="colorflipper.js"&gt;&lt;/script&gt;
&lt;/head&gt;
&lt;body&gt;

    &lt;nav&gt;
     &lt;div class = "nav-center"&gt;
   &lt;nav&gt;

&lt;/body&gt;

&lt;/html&gt;
</code></pre><p>Solution :
 Declare the source of script file "colorflipper.js" below or inside the body tag!</p>



```
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Color Flipper</title>
    <script src="colorflipper.js"></script>
</head>
<body>
    
    <nav>
     <div class = "nav-center">
   <nav>

</body>

</html>
``` 
Solution :
 Declare the source of script file "colorflipper.js" below or inside the body tag!

Uncaught TypeError: Cannot read properties of null (reading 'addEventListener')