fix monitor.stop() in some cases

server/model/monitor.js

@@ -309,14 +309,18 @@ class Monitor extends BeanModel {
 
             previousBeat = bean;
 
+            if (! this.isStop) {
                 this.heartbeatInterval = setTimeout(beat, this.interval * 1000);
             }
 
+        }
+
         beat();
     }
 
     stop() {
         clearTimeout(this.heartbeatInterval);
+        this.isStop = true;
     }
 
     /**