Subscription query initialResult throws NullPointerException when query handler returns null when using QueryGateway

Expecting subscriptionQuery initialResult empty Mono when query handler returns null. With Axon 3.3.6 we’re getting the following:

Caused by: java.lang.NullPointerException: The mapper returned a null value.
	at java.base/java.util.Objects.requireNonNull(Objects.java:246)
	at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:146)
	at org.axonframework.queryhandling.MonoSinkWrapper.success(MonoSinkWrapper.java:48)
	at java.base/java.util.concurrent.CompletableFuture.uniAcceptNow(CompletableFuture.java:753)
	at java.base/java.util.concurrent.CompletableFuture.uniAcceptStage(CompletableFuture.java:731)
	at java.base/java.util.concurrent.CompletableFuture.thenAccept(CompletableFuture.java:2108)
	at org.axonframework.queryhandling.SimpleQueryBus.lambda$subscriptionQuery$7(SimpleQueryBus.java:246)
	at org.axonframework.queryhandling.MonoWrapper.lambda$create$0(MonoWrapper.java:62)
	at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:54)
	at reactor.core.publisher.MonoMap.subscribe(MonoMap.java:55)
	at reactor.core.publisher.Mono.subscribe(Mono.java:3088)
	at reactor.core.publisher.Mono.subscribeWith(Mono.java:3196)
	at reactor.core.publisher.Mono.toFuture(Mono.java:3513)
	... 31 more

Test case to reproduce with Spring Boot 2.0.5:

import org.axonframework.queryhandling.QueryGateway;
import org.axonframework.queryhandling.QueryHandler;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.stereotype.Component;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.concurrent.ExecutionException;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNull;

@RunWith(SpringRunner.class)
@SpringBootTest
@SpringBootApplication
public class AxonApplicationTests {

    @Autowired
    QueryGateway queryGateway;

    @Test
    public void testNullReturnFromQueryHandler() throws ExecutionException, InterruptedException {
        String notFound = queryGateway.subscriptionQuery(new MyQuery("not found"), String.class, String.class)
                .initialResult()
                .toFuture().get();

        assertNull(notFound);
    }

    @Test
    public void testNonNullReturnFromQueryHandler() throws ExecutionException, InterruptedException {
        String found = queryGateway.subscriptionQuery(new MyQuery("found"), String.class, String.class)
                .initialResult()
                .toFuture().get();

        assertEquals(found, "found");
    }
}

class MyQuery {
    String param;

    public MyQuery(String param) {
        this.param = param;
    }
}

@Component
class MyQueryHandlerProjection {
    @QueryHandler
    public String handle(MyQuery query) {
        return "found".equals(query.param) ? "found" : null;
    }
}

[smcvb]

This issue has been resolved in commit 7be1c5c, hence closing. Thanks for notifying us of this @leowentzel!

[leowentzel]

Upgraded to 3.4 and it works! Well done!