title: it can not access popup.js file after creating a chrome extension
body: <p><strong>manifest.json</strong></p>
  
  <pre><code>{
    "name": "Summer",
    "version": "1.0",
    "manifest_version": 2,
    "description": "This is an addition extension",
    "browser_action": {
      "default_icon": "icon.png",
      "default_popup": "popup.html"
    }
  }
  </code></pre>
  
  <p><strong>popup.html</strong></p>
  
  <pre><code>&lt;!doctype html&gt;
  &lt;html&gt;
    &lt;head&gt;
      &lt;title&gt;Getting Started Extension's Popup&lt;/title&gt;
  
      &lt;!-- JavaScript and HTML must be in separate files for security. --&gt;
      &lt;script src="popup.js"&gt;&lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
          &lt;form name="form"&gt;
              &lt;div id="sayi1"&gt;Sayı 1 :    &lt;input type = "text" name="deger1"&gt;&lt;/div&gt; 
              &lt;div id="sayi2"&gt;Sayı 2 :    &lt;input type = "text" name="deger2"&gt;&lt;/div&gt; 
              &lt;div id="sonuc"&gt;Sonuç :     &lt;input type = "text" name="cevap"&gt;&lt;/div&gt;
              &lt;div id="button"&gt;&lt;input type="button" value="Hesapla" onclick="hesaplama()" /&gt;&lt;/div&gt;
          &lt;/form&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p><strong>popup.js</strong></p>
  
  <pre><code>function hesaplama()
  {
  var sayi1 = window.document.form.deger1.value;
  var sayi2 = window.document.form.deger2.value;
  var toplam = parseFloat(sayi1) + parseFloat(sayi2) ;
  window.document.form.cevap.value = toplam; 
  }
  </code></pre>
  
  <p>When i load this extension, i can see normally. But when i filled the deger1 and deger2 textbox and clicked the button, function is not working, in the sonuc textbox (result textbox) is null.
  How can i fix it? I am new at creating chrome extensions. Thanks for your help.</p>