When you declare RuleUnitData in DRL, DataStore can work without initialization because generated DTO has DataStore initialization (= DataSource.createStore()). But SingletonStore doesn't work because generated DTO doesn't have SingletonStore initialization (DataSource.createSingleton()).

Hello.drl

declare Hello extends RuleUnitData
  // result : SingletonStore<Result> = DataSource.createSingleton() // This works
  result : SingletonStore<Result> // This doesn't work
  messages : DataStore<String>
end

HelloDTO.java

    public Hello get() {
        Hello unit = new Hello();
        {
            org.kie.kogito.rules.DataStore<java.lang.String> messages = org.kie.kogito.rules.DataSource.createStore();
            unit.setMessages(messages);
            this.messages.forEach(unit.getMessages()::add);
        }
        {
            unit.getResultList().set(this.resultList);
        }
        return unit;
    }

It fails with NPE when accessed REST endpoint.

Caused by: java.lang.NullPointerException
	at org.kie.kogito.examples.HelloDTO.get(HelloDTO.java:9)
	at org.kie.kogito.examples.HelloQueryHelloEndpoint.executeQuery(HelloQueryHelloEndpoint.java:31)

If you explicitly initialize in the DRL (SingletonStore<Result> = DataSource.createSingleton()), it works. So this issue is not serious. Just to align with DataStore behaviour.

Note: however, overwriting in DTO may lose flexibility. See https://issues.redhat.com/browse/KOGITO-2029