Nifi Custom Processor - transfer relationship not specified

[#1]

I am building a custom processor that publishes a message to an internal system. I am getting an error that the "transfer relationship is not specified", and I haven't been able to figure out why. As far as I can see I am following what I have seen online and in other processors. Here is the code:

    public void onTrigger(final ProcessContext context, final ProcessSession session) throws ProcessException {
        FlowFile flowFile = session.get();
        if ( flowFile == null ) {
            return;
        }
        final AtomicReference<String> contentsRef = new AtomicReference<>(null);
        session.read(flowFile, (inputStream) -> {
            final String content = IOUtils.toString(inputStream, StandardCharsets.UTF_8);
            contentsRef.set(content);
        });
        try {
            final ValidatedEventStoreProducer producer = initializeProducer(context, session, flowFile);
            producer.publishEvent(contentsRef.get(), Topics.IQ_EVENT_GENERAL, TenantId.fromValidated(flowFile.getAttribute("tenantid")));
        } catch (Exception exception) {
            session.transfer(flowFile, REL_FAILURE);
            return;
        }
        session.transfer(flowFile, REL_SUCCESS);
    }

None of the operations return an updated flowFile so it is the right one I am sending. To test that I tried the following code:

public void onTrigger(final ProcessContext context, final ProcessSession session) throws ProcessException {
        FlowFile flowFile = session.get();
        if ( flowFile == null ) {
            return;
        }
        session.transfer(flowFile, REL_SUCCESS);
    }

and even:

public void onTrigger(final ProcessContext context, final ProcessSession session) throws ProcessException {
        FlowFile flowFile = session.get();
        if ( flowFile == null ) {
            return;
        }
        session.remove(flowFile);
    }

I still get " transfer relationship not specified".

I have my relationship set up like this:

public static final Relationship REL_SUCCESS = new Relationship.Builder()
            .name("success")
            .description("Files that are successfully send will be transferred to success")
            .build();
    public static final Relationship REL_FAILURE = new Relationship.Builder()
            .name("failure")
            .description("Files that fail to send will transferred to failure")
            .build();

These are added to the relationships Set.

If anyone could give me some advice and/or let me know where I'm going wrong I would really appreciate that! Thanks!