Fix WebFlux default error view for null exception messages

This commit prevents NullPointerExceptions when the default HTML error
view is being rendered with a `null` message.

Fixes gh-11677

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/AbstractErrorWebExceptionHandler.java

@@ -195,9 +195,13 @@ public abstract class AbstractErrorWebExceptionHandler
 				.append(HtmlUtils.htmlEscape(error.get("error").toString()))
 				.append(", status=")
 				.append(HtmlUtils.htmlEscape(error.get("status").toString()))
-				.append(").</div>").append("<div>")
-				.append(HtmlUtils.htmlEscape(error.get("message").toString()))
-				.append("</div>").append("</body></html>");
+				.append(").</div>");
+		if (error.get("message") != null) {
+			builder.append("<div>")
+					.append(HtmlUtils.htmlEscape(error.get("message").toString()))
+					.append("</div>");
+		}
+		builder.append("</body></html>");
 		return responseBody.syncBody(builder.toString());
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandlerIntegrationTests.java

@@ -240,6 +240,23 @@ public class DefaultErrorWebExceptionHandlerIntegrationTests {
 				});
 	}
 
+	@Test
+	public void testExceptionWithNullMessage() throws Exception {
+		this.contextRunner
+				.withPropertyValues("spring.mustache.prefix=classpath:/unknown/")
+				.run((context) -> {
+					WebTestClient client = WebTestClient.bindToApplicationContext(context)
+							.build();
+					String body = client.get().uri("/notfound").accept(MediaType.TEXT_HTML)
+							.exchange().expectStatus()
+							.isEqualTo(HttpStatus.NOT_FOUND).expectHeader()
+							.contentType(MediaType.TEXT_HTML).expectBody(String.class)
+							.returnResult().getResponseBody();
+					assertThat(body).contains("Whitelabel Error Page")
+							.contains("type=Not Found, status=404");
+				});
+	}
+
 	@Test
 	public void responseCommitted() throws Exception {
 		this.contextRunner.run((context) -> {