fix null pointer exception

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@zalando.de>

opentracing-spring-extension/opentracing-spring-webflux-extension/src/main/java/org/zalando/opentracing/spring/webflux/extension/ErrorSpanDecorator.java

@@ -1,19 +1,19 @@
 package org.zalando.opentracing.spring.webflux.extension;
 
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+import java.util.function.Predicate;
+
 import io.opentracing.Span;
 import io.opentracing.log.Fields;
 import io.opentracing.tag.Tags;
 import lombok.AllArgsConstructor;
 import org.apiguardian.api.API;
+import static org.apiguardian.api.API.Status.*;
 import org.springframework.http.HttpStatus;
 import org.springframework.web.server.ServerWebExchange;
 
-import java.util.HashMap;
-import java.util.Map;
-import java.util.function.Predicate;
-
-import static org.apiguardian.api.API.Status.EXPERIMENTAL;
-
 @API(status = EXPERIMENTAL)
 @AllArgsConstructor
 public final class ErrorSpanDecorator
@@ -30,7 +30,8 @@ public void onResponse(
             final ServerWebExchange exchange,
             final Span span) {
 
-        if (predicate.test(exchange.getResponse().getStatusCode())) {
+        HttpStatus statusCode = exchange.getResponse().getStatusCode();
+        if (statusCode == null || predicate.test(statusCode)) {
             span.setTag(Tags.ERROR, true);
         }
     }

opentracing-spring-extension/opentracing-spring-webflux-extension/src/test/java/org/zalando/opentracing/spring/webflux/extension/ErrorSpanDecoratorTest.java

@@ -0,0 +1,23 @@
+package org.zalando.opentracing.spring.webflux.extension;
+
+import io.opentracing.mock.MockSpan;
+import io.opentracing.mock.MockTracer;
+import static org.assertj.core.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+import static org.mockito.Mockito.*;
+import org.springframework.http.server.reactive.ServerHttpResponse;
+import org.springframework.web.server.ServerWebExchange;
+
+class ErrorSpanDecoratorTest {
+
+    @Test
+    void onTagNullStatusCode() {
+        ServerWebExchange exchange = mock(ServerWebExchange.class);
+        when(exchange.getResponse()).thenReturn(mock(ServerHttpResponse.class));
+
+        MockSpan span = new MockTracer().buildSpan("span").start();
+        new ErrorSpanDecorator().onResponse(exchange, span);
+
+        assertThat(span.tags().get("error")).isEqualTo(true);
+    }
+}