To add to Henri’s answer, if the HttpResponse is a Mendix proxy object, you need to return its iMendixObject, not null. For example...
return httpResponse.getMendixObject();
You need to return the same data type as you specified for your method and action. You expect an implementation of the interface IMendixObject, but you are returning null.
What do you want to achieve? If you want to implement REST API calls, Mendix has an easy to use toolbox to implement it. Java Actions are only recommended if Mendix has no workable support to achieve your goal. It’s also important that you know what you’re doing.