Consistent nullability for concurrent result

(cherry picked from commit b92877990d)

spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -159,6 +159,7 @@ public final class WebAsyncManager {
 	 * concurrent handling raised one.
 	 * @see #clearConcurrentResult()
 	 */
+	@Nullable
 	public Object getConcurrentResult() {
 		return this.concurrentResult;
 	}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -48,6 +48,7 @@ import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
 import org.springframework.lang.Nullable;
 import org.springframework.ui.ModelMap;
+import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.ReflectionUtils.MethodFilter;
 import org.springframework.web.accept.ContentNegotiationManager;
@@ -872,7 +873,9 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
 
 		if (asyncManager.hasConcurrentResult()) {
 			Object result = asyncManager.getConcurrentResult();
-			mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
+			Object[] resultContext = asyncManager.getConcurrentResultContext();
+			Assert.state(resultContext != null && resultContext.length > 0, "Missing result context");
+			mavContainer = (ModelAndViewContainer) resultContext[0];
 			asyncManager.clearConcurrentResult();
 			LogFormatUtils.traceDebug(logger, traceOn -> {
 				String formatted = LogFormatUtils.formatValue(result, !traceOn);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ServletInvocableHandlerMethod.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -200,7 +200,7 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 	 * actually invoking the controller method. This is useful when processing
 	 * async return values (e.g. Callable, DeferredResult, ListenableFuture).
 	 */
-	ServletInvocableHandlerMethod wrapConcurrentResult(Object result) {
+	ServletInvocableHandlerMethod wrapConcurrentResult(@Nullable Object result) {
 		return new ConcurrentResultHandlerMethod(result, new ConcurrentResultMethodParameter(result));
 	}
 
@@ -215,7 +215,7 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 
 		private final MethodParameter returnType;
 
-		public ConcurrentResultHandlerMethod(final Object result, ConcurrentResultMethodParameter returnType) {
+		public ConcurrentResultHandlerMethod(@Nullable Object result, ConcurrentResultMethodParameter returnType) {
 			super((Callable<Object>) () -> {
 				if (result instanceof Exception exception) {
 					throw exception;
@@ -279,7 +279,7 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 
 		private final ResolvableType returnType;
 
-		public ConcurrentResultMethodParameter(Object returnValue) {
+		public ConcurrentResultMethodParameter(@Nullable Object returnValue) {
 			super(-1);
 			this.returnValue = returnValue;
 			this.returnType = (returnValue instanceof CollectedValuesList cvList ?