Why do manual AJAX calls from a JSF portlet results in NullPointerException in JPP 6.1?

Issue

We have a JSF portlet which makes requests to a JSF PhaseListener using manual javascript. When installing this portlet into JPP, we have NullPointerException (NPE) in the logs. How can we solve that?

ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/FJC_iBOT_Quiz_Portlet].[Faces Servlet]] (http-/0.0.0.0:8080-1) JBWEB000236: Servlet.service() for servlet Faces Servlet threw exception: java.lang.NullPointerException
    at org.jboss.portletbridge.context.exception.PortletExceptionHandler.handle(PortletExceptionHandler.java:64) [portletbridge-impl-3.2.1.Final-redhat-4.jar:3.2.1.Final-redhat-4]
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:119) [jsf-impl-2.1.19-redhat-1.jar:2.1.19-redhat-1]
    at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:116) [jsf-impl-2.1.19-redhat-1.jar:2.1.19-redhat-1]
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118) [jsf-impl-2.1.19-redhat-1.jar:2.1.19-redhat-1]
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593) [jboss-jsf-api_2.1_spec-2.1.19.1.Final-redhat-1.jar:2.1.19.1.Final-redhat-1]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:295) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:149) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:169) [jboss-as-web-7.2.1.Final-redhat-10.jar:7.2.1.Final-redhat-10]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:145) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:97) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:102) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:336) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:653) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:920) [jbossweb-7.2.2.Final-redhat-1.jar:7.2.2.Final-redhat-1]
    at java.lang.Thread.run(Unknown Source) [rt.jar:1.7.0_45]