Spring NullPointerException When Handling 407 with JdkClientHttpConnector in WebClient

When using a WebClient that is configured with a proxy and using the JdkClientHttpConnector, when a 407 Proxy Authentication Required error is returned then a NullPointerException is thrown when trying to read the response body.

Sample Code

    @Test
    void proxyError() {
        final HttpClient httpClient = HttpClient.newBuilder()
                .proxy(ProxySelector.of(new InetSocketAddress("my-authenticated,proxy.net", 9443)))
                .build();
        final  WebClient webClient = WebClient.builder()
                .baseUrl("https://start.spring.io/actuator/info")
                .clientConnector(new JdkClientHttpConnector(httpClient))
                .build();
        webClient.get().uri("/").exchangeToMono(response -> {
            if (response.statusCode() == HttpStatus.PROXY_AUTHENTICATION_REQUIRED) {
                return response.bodyToMono(String.class)
                        .flatMap(body -> Mono.error(new RuntimeException("Proxy Auth Required. " + body)));
            }
            return response.bodyToMono(String.class);
        }).block();
    }

Stacktrace

java.lang.NullPointerException: Cannot invoke "java.util.concurrent.Flow$Publisher.subscribe(java.util.concurrent.Flow$Subscriber)" because "this.pub" is null

    at reactor.adapter.JdkFlowAdapter$FlowPublisherAsFlux.subscribe(JdkFlowAdapter.java:68)
    Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ Body from GET https://start.spring.io/actuator/info/ [DefaultClientResponse]
Original Stack Trace:
        at reactor.adapter.JdkFlowAdapter$FlowPublisherAsFlux.subscribe(JdkFlowAdapter.java:68)
        at reactor.core.publisher.InternalFluxOperator.subscribe(InternalFluxOperator.java:68)
        at reactor.core.publisher.FluxPublish.connect(FluxPublish.java:106)
        at reactor.core.publisher.FluxAutoConnect.subscribe(FluxAutoConnect.java:62)
        at reactor.core.publisher.Flux.subscribe(Flux.java:8891)
        at org.springframework.http.client.reactive.AbstractClientHttpResponse$SingleSubscriberPublisher.subscribe(AbstractClientHttpResponse.java:112)
        at reactor.core.publisher.FluxSource.subscribe(FluxSource.java:71)
        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76)
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
        at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107)
        at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onNext(FluxDoFinally.java:113)
        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onNext(MonoPeekTerminal.java:180)
        at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
        at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:74)
        at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:122)
        at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79)
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:200)
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:200)
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:294)
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:188)
        at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:122)
        at reactor.core.publisher.MonoCompletionStage$MonoCompletionStageSubscription.apply(MonoCompletionStage.java:121)
        at reactor.core.publisher.MonoCompletionStage$MonoCompletionStageSubscription.apply(MonoCompletionStage.java:67)
        at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934)
        at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911)
        at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
        at java.base/java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:614)
        at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:844)
        at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483)
        at java.base/java.util.concurrent.ForkJoinTask.doExec$$$capture(ForkJoinTask.java:387)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1310)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1841)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1806)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104)
        at reactor.core.publisher.Mono.block(Mono.java:1779)
        at com.example.webclient_release_body.WebclientReleaseBodyApplicationTests.proxyError(WebclientReleaseBodyApplicationTests.java:32)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

Spring Framework Version: 6.2.12