A bit different issue comparing with
FACES-1843. In Liferay 6.2, portlet still works after FACES-1843 fix, but still will print an error.
This is caused by different RequestDispatcher used. For Liferay 6.2, Tomcat's one will print the error, while in Liferay 7, equinox won't print any error but just throw it up in the stack.