title: getting Exception : java.lang.IllegalArgumentException: cannot add to layout: constraint must be a string (or null)
body: <p>I am implementing browser kind of project and I am getting an exception.</p>
  
  <pre><code>import java.awt.BorderLayout;
  import java.awt.Dimension;
  import java.awt.GridBagConstraints;
  import java.awt.Insets;
  import javax.swing.JPanel;
  import javax.swing.JScrollPane;
  import javax.swing.SwingUtilities;
  import chrriis.dj.nativeswing.swtimpl.NativeInterface;
  import chrriis.dj.nativeswing.swtimpl.components.JWebBrowser;
  
  public class WebPageDisplay extends JPanel{
  
      public WebPageDisplay() {  
  
           super(new BorderLayout());  
           try{
         JPanel webBrowserPanel = new JPanel(new BorderLayout());   
         // webBrowserPanel.setBorder(BorderFactory.createTitledBorder("Native Web Browser component"));   
          final JWebBrowser webBrowser = new JWebBrowser();   
          webBrowser.setBarsVisible(false);   
          webBrowser.setStatusBarVisible(true);   
          webBrowserPanel.add(webBrowser, BorderLayout.CENTER);   
          add(webBrowserPanel, BorderLayout.CENTER);
                      webBrowser.navigate("www.google.com");
          JScrollPane scrollPane = new JScrollPane();   
          Dimension preferredSize = scrollPane.getPreferredSize();   
          preferredSize.height += 20;   
          scrollPane.setPreferredSize(preferredSize);   
  //      add(scrollPane, BorderLayout.NORTH);   
          add(scrollPane,new GridBagConstraints(0, 2, 2, 1, 1.0, 1.0, GridBagConstraints.CENTER, 
              GridBagConstraints.BOTH, new Insets(0, 0, 0, 0), 0, 0));   
  
           }catch(Exception e){
               e.printStackTrace();
  
          }
        }
  
      public static void main(String[] args){
          NativeInterface.open();
  
           SwingUtilities.invokeLater(new Runnable() {
  
                public void run() {
  
                   WebPageDisplay webDisplay = new WebPageDisplay();
  
                }
  
              });
  
      }
  }
  </code></pre>
  
  <p>Here is the exception.</p>
  
  <pre><code>java.lang.IllegalArgumentException: cannot add to layout: constraint must be a string (or null)
      at java.awt.BorderLayout.addLayoutComponent(BorderLayout.java:409)
      at java.awt.Container.addImpl(Container.java:1074)
      at java.awt.Container.add(Container.java:927)
      at WebPageDisplay.&lt;init&gt;(PBXPageDisplay.java:35)
      at WebPageDisplay$1.run(PBXPageDisplay.java:51)
      at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
      at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:677)
      at java.awt.EventQueue.access$000(EventQueue.java:85)
      at java.awt.EventQueue$1.run(EventQueue.java:638)
      at java.awt.EventQueue$1.run(EventQueue.java:636)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
      at java.awt.EventQueue.dispatchEvent(EventQueue.java:647)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
      at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
      at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
  </code></pre>