getting style error for display with JS but works smoothly in jsfiddle

I have the code below. My desired outcome is exactly how it is in jsfiddle: works perfectly! In my browser, I get these two errors that differentiate it:

code:

http://jsfiddle.net/pbvwjL94/1/

GET http://localhost/favicon.ico 404 (Not Found)

realjs.js:236 Uncaught TypeError: Cannot read property 'style' of null
    at showForm (realjs.js:236)
    at HTMLSelectElement.onchange (testwriting.php:57)

In my browser, the writing sections work perfectly and the reading section works when I choose science. However, the error still appears. The other sections just do not work.

I know it is a lot of code but I have been scouring the web for answers for so long and nobody has been able to help me.

I really need it guys

Thanks

As asked, here is my element with id f9

<div class="form-group" name="skill5" id="f9" style="display:none">
      <select class="form-control" name="skill5" id="readingsocialstudies" onchange="showForm()" data-required>
        <option value="">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-Select Your Skill-</option>
        <option value="Big Picture">Big Picture</option>
        <option value="Attention to Detail">Attention to Detail</option>
        <option value="Inference">Making Inferences</option>
        <option value="Function">Identifying Function</option>
        <option value="Vocabulary In Context">Vocabulary In Context</option>
        <option value="Analogy">Drawing Analogies</option>
        <option value="Author Technique">Author Technique</option>
        <option value="Evidence Support">Supporting with Evidence</option>
        <option value="Data Reasoning">Data Reasoning</option>
      </select>   
      <br style="line-height:10px;">
      <select class="form-control" name="passageid1" data-required>
        <option value="">&nbsp;&nbsp;&nbsp;-Select Your Passage ID-</option>
          <?php 
            $sql = mysqli_query($connect, "SELECT ID FROM excerptlist");
            while ($row = $sql->fetch_assoc()){
              $ID = $row['ID'];
              echo "<option value='".$ID."'>" . $ID . "</option>";
            }
          ?>
      </select>  
    </div>