title: GWT UiBinder HTML not showing up
body: <p>working on my first GWT project and having some trouble with the UiBinder. When I load my module not even HTML is showing up, I just see a blank page.
  Anyone know why? What am I leaving out? </p>
  
  <p>I haven't grasped the concept of panels yet, is RootPanel the ScrollPanel? Or mainPanel because I gave it an identifier?</p>
  
  <p>edit: I've run into the error: this panel does not support no-arg add. The error doesn't say which one though. Any tips on how to get the layout I want?</p>
  
  <p>Thanks in advance.</p>
  
  <p>UiBinder class DoraTheExplora.ui.xml:</p>
  
  <pre><code>&lt;g:ScrollPanel&gt;
      &lt;g:VerticalPanel ui:field="mainPanel"&gt;
      &lt;!-- Header of Dora website --&gt;
      &lt;g:HorizontalPanel ui:field="headerPanel"&gt;
      &lt;g:HTMLPanel styleName="{resource.style.siteHeader}"&gt;
          Welcome to Dora!
      &lt;/g:HTMLPanel&gt;
  ...
  </code></pre>
  
  <p>EntryPoint class :</p>
  
  <pre><code>...
  public void onModuleLoad() {
      RootLayoutPanel.get().add(new DoraTheExplora());
  }
  ...
  </code></pre>
  
  <p>DoraTheExplora class:</p>
  
  <pre><code>...
  public class DoraTheExplora extends Composite {
  
  // UIBinder instance.
  @UiTemplate("DoraTheExplora.ui.xml")
  interface DoraTheExploraUiBinder extends UiBinder&lt;Widget, DoraTheExplora&gt; {
  }
  private static DoraTheExploraUiBinder uiBinder = GWT.create(DoraTheExploraUiBinder.class);
  
  @UiField
  VerticalPanel mainPanel;
  @UiField
  HorizontalPanel headerPanel;
  ...
  
  public DoraTheExplora() {
      initWidget(uiBinder.createAndBindUi(this));
  ...
      }
  }
  </code></pre>