title: GWT, RootPanel.get() is null
body: <p>I'm following a beginner's tutorial at <a href="http://www.tutorialspoint.com/gwt/gwt_style_with_css.htm" rel="nofollow" title="tutorialspoint">http://www.tutorialspoint.com/gwt/gwt_style_with_css.htm</a>. 
  The code for the HelloWorld.html file is: </p>
  
  <pre><code>&lt;html&gt;
  &lt;head&gt;
  &lt;title&gt;Hello World&lt;/title&gt;
     &lt;link rel="stylesheet" href="HelloWorld.css"/&gt;
     &lt;script language="javascript" src="helloworld/helloworld.nocache.js"&gt;
     &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  
  &lt;div id="mytext"&gt;&lt;h1&gt;Hello, World!&lt;/h1&gt;&lt;/div&gt;
  &lt;div id="gwtGreenButton"&gt;&lt;/div&gt;
  &lt;div id="gwtRedButton"&gt;&lt;/div&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>In the HelloWorld.java (extending EntryPoint), I have: </p>
  
  <pre><code>RootPanel.get("gwtGreenButton").add(Btn1);
  RootPanel.get("gwtRedButton").add(Btn2);
  </code></pre>
  
  <p>The 2 lines: RootPanel.get() always result in null. I don't know what happen, what to check? (too bad the site doesn't have a comment/discussion section)</p>
  
  <p>Thanks.</p>