[SignalR] [Java] Fix NPE when closing hub connection during negotiation (#62319)

Author: Sejsel

src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/HubConnection.java

@@ -445,7 +445,8 @@ private Completable stop(String errorMessage) {
         CompletableSubject subject = CompletableSubject.create();
         startTask.onErrorComplete().subscribe(() ->
         {
-            Completable stop = connectionState.transport.stop();
+            Transport transport = connectionState.transport;
+            Completable stop = (transport != null) ? transport.stop() : Completable.complete();
             stop.subscribe(() -> subject.onComplete(), e -> subject.onError(e));
         });
 

src/SignalR/clients/java/signalr/test/src/main/java/com/microsoft/signalr/HubConnectionTest.java

@@ -2783,6 +2783,32 @@ public void negotiateSentOnStart() {
         assertEquals("http://example.com/negotiate?negotiateVersion=1", sentRequests.get(0).getUrl());
     }
 
+    @Test
+    public void closeWithPendingNegotiate() {
+        SingleSubject<HttpResponse> responseSubject = SingleSubject.create();
+
+        TestHttpClient client = new TestHttpClient()
+                .on("POST", (req) -> responseSubject);
+
+        HubConnection hubConnection = HubConnectionBuilder
+                .create("http://example.com")
+                .withHttpClient(client)
+                .build();
+
+        Completable start = hubConnection.start();
+        assertEquals(HubConnectionState.CONNECTING, hubConnection.getConnectionState());
+
+        Completable stop = hubConnection.stop();
+
+        responseSubject.onSuccess(new HttpResponse(404, "", TestUtils.emptyByteBuffer));
+        stop.timeout(3, TimeUnit.SECONDS).blockingAwait();
+        assertEquals(HubConnectionState.DISCONNECTED, hubConnection.getConnectionState());
+
+        HttpRequestException exception = assertThrows(HttpRequestException.class, () -> start.blockingAwait(10, TimeUnit.SECONDS));
+        assertEquals("Unexpected status code returned from negotiate: 404 .", exception.getMessage());
+        assertEquals(404, exception.getStatusCode());
+    }
+
     @Test
     public void negotiateThatRedirectsForeverFailsAfter100Tries() {
         TestHttpClient client = new TestHttpClient().on("POST", "http://example.com/negotiate?negotiateVersion=1",