"NullPointerException" when accessing Import/Export gadget from my custom ecmdemo based container in EPP 5

Issue

When I try to access the Export/Import gadget from my ecmdemo based portal container I have the following NullPointerException:

ERROR [portal:UIPortletLifecycle] (http-127.0.0.1-8080-7) Portlet render threw an exception
org.exoplatform.services.portletcontainer.PortletContainerException: java.lang.NullPointerException
    (...)   
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:451)
    at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NullPointerException
    at org.exoplatform.portal.webui.application.ModelAdapter$2.getPortletContext(ModelAdapter.java:204) 
org.exoplatform.portal.webui.application.UIPortletLifecycle.processRender(UIPortletLifecycle.java:212)
    ... 171 more****

How to solve that issue?