Improve null-safety of module/spring-boot-web-server

See gh-46926

module/spring-boot-web-server/src/main/java/org/springframework/boot/web/server/reactive/context/ReactiveWebServerApplicationContext.java

@@ -102,11 +102,11 @@ public class ReactiveWebServerApplicationContext extends GenericReactiveWebAppli
 			ReactiveWebServerFactory webServerFactory = getWebServerFactory(webServerFactoryBeanName);
 			createWebServer.tag("factory", webServerFactory.getClass().toString());
 			boolean lazyInit = getBeanFactory().getBeanDefinition(webServerFactoryBeanName).isLazyInit();
-			this.serverManager = new WebServerManager(this, webServerFactory, this::getHttpHandler, lazyInit);
+			serverManager = new WebServerManager(this, webServerFactory, this::getHttpHandler, lazyInit);
+			this.serverManager = serverManager;
 			getBeanFactory().registerSingleton("webServerGracefulShutdown",
-					new WebServerGracefulShutdownLifecycle(this.serverManager.getWebServer()));
-			getBeanFactory().registerSingleton("webServerStartStop",
-					new WebServerStartStopLifecycle(this.serverManager));
+					new WebServerGracefulShutdownLifecycle(serverManager.getWebServer()));
+			getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(serverManager));
 			createWebServer.end();
 		}
 		initPropertySources();

module/spring-boot-web-server/src/main/java/org/springframework/boot/web/server/servlet/context/ServletWebServerApplicationContext.java

@@ -187,12 +187,12 @@ public class ServletWebServerApplicationContext extends GenericWebApplicationCon
 			StartupStep createWebServer = getApplicationStartup().start("spring.boot.webserver.create");
 			ServletWebServerFactory factory = getWebServerFactory();
 			createWebServer.tag("factory", factory.getClass().toString());
-			this.webServer = factory.getWebServer(getSelfInitializer());
+			webServer = factory.getWebServer(getSelfInitializer());
+			this.webServer = webServer;
 			createWebServer.end();
 			getBeanFactory().registerSingleton("webServerGracefulShutdown",
-					new WebServerGracefulShutdownLifecycle(this.webServer));
-			getBeanFactory().registerSingleton("webServerStartStop",
-					new WebServerStartStopLifecycle(this, this.webServer));
+					new WebServerGracefulShutdownLifecycle(webServer));
+			getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, webServer));
 		}
 		else if (servletContext != null) {
 			try {