Fix visualizer bug (#926)

*  Fix unregistering WS handler

*  Fix concurrent map read/write

plugins/dashboard/frontend/src/app/stores/NodeStore.ts

@@ -188,7 +188,7 @@ export class NodeStore {
 
     unregisterHandlers = () => {
         unregisterHandler(WSMsgType.Status);
-        registerHandler(WSMsgType.MPSMetrics, this.updateLastMPSMetric);
+        unregisterHandler(WSMsgType.MPSMetrics);
         unregisterHandler(WSMsgType.NeighborStats);
         unregisterHandler(WSMsgType.TipsMetrics);
         this.updateCollecting(false);

plugins/dashboard/visualizer.go

@@ -134,9 +134,10 @@ func runVisualizer() {
 func setupVisualizerRoutes(routeGroup *echo.Group) {
 	routeGroup.GET("/visualizer/history", func(c echo.Context) (err error) {
 		msgHistoryMutex.RLock()
+		defer msgHistoryMutex.RUnlock()
+
 		cpyHistory := make([]*tangle.Message, len(msgHistory))
 		copy(cpyHistory, msgHistory)
-		msgHistoryMutex.RUnlock()
 
 		var res []vertex
 		for _, msg := range cpyHistory {