Fixed Failure of Enhanced Kafka Interceptor to Acquire SkyWalking DynamicField (#2470)

apm-sniffer/apm-sdk-plugin/kafka-v1-plugin/src/main/java/org/apache/skywalking/apm/plugin/kafka/v1/CallbackInterceptor.java

@@ -34,7 +34,9 @@ public class CallbackInterceptor implements InstanceMethodsAroundInterceptor {
 
         //Get the SnapshotContext
         ContextSnapshot contextSnapshot = (ContextSnapshot)objInst.getSkyWalkingDynamicField();
-        ContextManager.continued(contextSnapshot);
+        if (null != contextSnapshot) {
+            ContextManager.continued(contextSnapshot);
+        }
     }
 
     @Override