fix (#3057)

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java

@@ -157,6 +157,10 @@ public class ContextManager implements BootService {
         return get().awaitFinishAsync();
     }
 
+    /**
+     * If not sure has the active span, use this method, will be cause NPE when has no active span,
+     * use ContextManager::isActive method to determine whether there has the active span.
+     */
     public static AbstractSpan activeSpan() {
         return get().activeSpan();
     }

apm-sniffer/optional-plugins/customize-enhance-plugin/src/main/java/org/apache/skywalking/apm/plugin/customize/interceptor/BaseInterceptorMethods.java

@@ -92,7 +92,7 @@ class BaseInterceptorMethods {
     }
 
     void handleMethodException(Throwable t) {
-        if (ContextManager.activeSpan() != null) {
+        if (ContextManager.isActive()) {
             ContextManager.activeSpan().errorOccurred().log(t);
         }
     }

apm-sniffer/optional-plugins/optional-spring-plugins/optional-spring-cloud/gateway-2.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/spring/cloud/gateway/v2/FilteringWebHandlerInterceptor.java

@@ -42,9 +42,8 @@ public class FilteringWebHandlerInterceptor implements InstanceMethodsAroundInte
         ServerWebExchange exchange = (ServerWebExchange) allArguments[0];
         Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
 
-        AbstractSpan span = ContextManager.activeSpan();
-        if (span != null) {
-            span.tag("route", route.getId());
+        if (ContextManager.isActive()) {
+            ContextManager.activeSpan().tag("route", route.getId());
         }
     }