why portlet application url is null.

I am developing a portal application using vaadin , spring. On one of the portlet page i need to pop out a native window.
But i am always getting ‘null’ as application URL and a exception is thrown.
After looking into code for AbstractApplicationPortlet#startApplication method , i can see that URL is null.

So now how can i open a blank window while using portlet.

vaadin 6.6.X
liferay portal 6.0.5

When using Portlet 2.0 (JSR-286) portlets, the portlet itself can only be accessed via the portlet container (portal). There does not exist any servlet that could be accessed directly with a static URL for the application.

All URLs used within the portlet must be created by the portlet container, using RenderResponse.createUrl() or ResourceResponse.createUrl(). Beyond what the specification covers, there are many portlet container specific details to be aware of - e.g. such URLs might contain any parameters you specify in some encoded format, or the parameters might be kept on the server side only. Also, various portlet containers may provide additional (non-standard) APIs.

In theory, you could also create a servlet configuration in the same WAR as the portlet, but that easily gets complicated.