GWT: execution stops when adding a Composite widget to RootPanel

I have an abstract class extending Composite (AbstractWhiteBoard). Then I have a concrete class extending AbstractWhiteBoard. When I instantiate the concrete class and try to add it to the RootPanel, the program simply stops executing. There is no error or any output to direct me to a log file. I have no idea what is going wrong.

Here is my abstract class:

public abstract class AbstractWhiteBoard extends Composite {
     /*
      * FIELDS
      */
     protected HorizontalPanel WhiteBoardWrapperPanel;

     public AbstractWhiteBoard( ) {
          WhiteBoardWrapperPanel = new HorizontalPanel();
          WhiteBoardWrapperPanel.setStyleName("WhiteBoard-Wrapper");
          initWidget(WhiteBoardWrapperPanel);
     }

     /*
      * ABSTRACT PUBLIC METHODS
      */
     abstract public void addNotecard( Notecard nc );
     abstract public void addPostit( Postit postit );

     /*
      * ABSTRACT PROTECTED HELPER METHODS
      */
     abstract protected void registerDragDropControllers();
 }

And here is my concrete implementation class:

public class ConcreteWhiteBoard extends AbstractWhiteBoard {

/*
 * CONTSTRUCTORS
 */
public ConcreteWhiteBoard() {
    super();
}


/*
 * PUBLIC METHOD OVERRIDES
 */
@Override
public void addNotecard(Notecard nc) {
    // TODO Auto-generated method stub

}

@Override
public void addPostit(Postit postit) {
    // TODO Auto-generated method stub

}


/*
 * PRIVATE HELPER METHOD OVERRIDES
 */
@Override
protected void registerDragDropControllers() {
    // TODO Auto-generated method stub
}
}

So, what is happening, is I have this code:

AbstractWhiteBoard wb = new ConcreteWhiteBoard();
RootPanel.get().add(wb);
Window.alert("wb added!");

But after I add wb to the RootPanel, execution stops. The alert statement never even gets called. There is no error and I don't see anything in the log.

Is there something wrong with having an abstract class that extends Composite? Or is it something entirely different that I am just not seeing? any help is greatly appreciated!