NPE after canceling the download of large file

To reproduce

• use the download link from the folder listing to retrieve a large file
• cancel the operation from your navigator
• refresh the page you got an "HTTP Status 500 - " with a traceback

java.lang.NullPointerException
org.apache.catalina.connector.Request.removeAttribute(Request.java:1354)
org.apache.catalina.connector.RequestFacade.removeAttribute(RequestFacade.java:514)
org.jboss.seam.mock.MockExternalContext$2.removeAttribute(MockExternalContext.java:274)
org.jboss.seam.mock.MockExternalContext$AttributeMap.remove(MockExternalContext.java:471)
org.jboss.seam.contexts.BasicContext.remove(BasicContext.java:73)
org.jboss.seam.Component.newInstance(Component.java:2070)