[PinpiontWeb] fix a ActiveThread concurrency problem 
TreeMap of ActiveThreadCountResponseAggregator is not thread safe
- local variable : activeThreadCountWorkerRepository

web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java
web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java
diff --git a/web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java b/web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java
index 749a8cc15438..2b114950fd78 100644
--- a/web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java
+++ b/web/src/main/java/com/navercorp/pinpoint/web/websocket/ActiveThreadCountHandler.java
@@ -40,6 +40,7 @@
 import org.springframework.web.socket.handler.TextWebSocketHandler;
 
 import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -59,7 +60,7 @@ public class ActiveThreadCountHandler extends TextWebSocketHandler implements Pi
     private final Object lock = new Object();
     private final AgentService agentSerivce;
     private final List<WebSocketSession> sessionRepository = new CopyOnWriteArrayList<>();
-    private final Map<String, PinpointWebSocketResponseAggregator> aggregatorRepository = new HashMap<>();
+    private final Map<String, PinpointWebSocketResponseAggregator> aggregatorRepository = new ConcurrentHashMap<>();
     private PinpointWebSocketMessageConverter messageConverter = new PinpointWebSocketMessageConverter();
 
     private static final String DEFAULT_REQUEST_MAPPING = "/agent/activeThread";
