[PinpiontWeb] fix a ActiveThread concurrency problem
If multiple threads invokes WebSocketSession's sendMessage method at the same time, ; 
then it is possible to makes IllegalStateException.

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 2b865789c33f..8da38eea614e 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
@@ -214,7 +214,11 @@ private void handleRequestMessage0(WebSocketSession webSocketSession, RequestMes
                     }
 
                     unbindingResponseAggregator(webSocketSession);
-                    bindingResponseAggregator(webSocketSession, applicationName);
+                    if (webSocketSession.isOpen()) {
+                        bindingResponseAggregator(webSocketSession, applicationName);
+                    } else {
+                        logger.warn("WebSocketSession is not opened. skip binding.");
+                    }
                 }
             }
         }
