Fix NPE when autoconfigured WebTestClient is used

When `management.server.port` != `server.port` and test is using the autoconfigured WebTestClient, a NPE is raised by `RoutePredicateHandlerMapping`

[related gh-2870]
Fixes gh-2872

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/handler/RoutePredicateHandlerMapping.java

@@ -77,6 +77,7 @@ public class RoutePredicateHandlerMapping extends AbstractHandlerMapping {
 	protected Mono<?> getHandlerInternal(ServerWebExchange exchange) {
 		// don't handle requests on management port if set and different than server port
 		if (this.managementPortType == DIFFERENT && this.managementPort != null
+				&& exchange.getRequest().getLocalAddress() != null
 				&& exchange.getRequest().getLocalAddress().getPort() == this.managementPort) {
 			return Mono.empty();
 		}

spring-cloud-gateway-server/src/test/java/org/springframework/cloud/gateway/handler/RoutePredicateHandlerMappingWithAutoconfiguredClientIntegrationTests.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2013-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.gateway.handler;
+
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.SpringBootConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.gateway.route.RouteLocator;
+import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
+import org.springframework.cloud.gateway.test.BaseWebClientTests;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.util.TestSocketUtils;
+import org.springframework.test.web.reactive.server.WebTestClient;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import static org.springframework.boot.test.context.SpringBootTest.WebEnvironment.RANDOM_PORT;
+
+@SpringBootTest(webEnvironment = RANDOM_PORT)
+@AutoConfigureWebTestClient
+public class RoutePredicateHandlerMappingWithAutoconfiguredClientIntegrationTests {
+
+	@Autowired
+	WebTestClient webTestClient;
+
+	@BeforeAll
+	static void beforeClass() {
+		int managementPort = TestSocketUtils.findAvailableTcpPort();
+		System.setProperty("management.server.port", String.valueOf(managementPort));
+	}
+
+	@AfterAll
+	static void afterClass() {
+		System.clearProperty("management.server.port");
+	}
+
+	@Test
+	void shouldReturnOk() {
+		this.webTestClient.get().uri("/get").exchange().expectStatus().isOk();
+	}
+
+	@EnableAutoConfiguration
+	@SpringBootConfiguration
+	@Import(BaseWebClientTests.DefaultTestConfig.class)
+	@RestController
+	public static class TestConfig {
+
+		@Value("${test.uri:http://httpbin.org:80}")
+		String uri;
+
+		@GetMapping("/get")
+		String get() {
+			return "hello";
+		}
+
+		@Bean
+		RouteLocator testRoutes(RouteLocatorBuilder builder) {
+			return builder.routes().route(predicateSpec -> predicateSpec.path("/get").uri(uri)).build();
+		}
+
+	}
+
+}