FileDownloader Push failed

Using Vaadin 7.3.7, while the appplication view is render, the application log following message :

java.lang.RuntimeException: Push failed
Caused by: com.vaadin.server.PaintException: Failed to serialize shared state for connector com.vaadin.server.FileDownloader (100): null
Caused by: org.json.JSONException

The application is using a custom resource component based on FileResource which create downloaded file on demande.
What’s the root cause ?

I get the solution by reading FileDownloader class comments.

The custom resource class has been changed to use FileResource as base class.