from the stack trace, it looks like one of these calls to addChild is passing in null:

    public void setContent(Canvas newContent) {
        if (newContent instanceof HasViewName) {
            LocatableVLayout decoratedContent = decorateWithTitleBar(((HasViewName) newContent).getViewName(),
                newContent);
            contentCanvas.addChild(decoratedContent);
        } else {
            contentCanvas.addChild(newContent);
        }