Major Description
I add a component to its container when the page is initialized, BEFORE
it is rendered and before page model finished to initialize.
if log4j is configured to enable wicket debug, the container toString()
is called.
This method invokes isVisible() of the container.
I have overridden isVisible(), to query the page model before deciding
whether the component is actually visible.
I assumed this method is called when the component is rendered, and by
that time the model is completely initialized.
However, since it was called before component rendering, my code failed
and the page failed to be created.
To sum up, toString() of Component should be safe.