imporve the code style

modules/history-track/TrackController.js

     this._viewer = viewer
     this._cache = {}
     this._delegete = new Cesium.CustomDataSource('history-track-layer')
-    viewer.dataSources.add(this._delegete)
+    this._viewer.dataSources.add(this._delegete)
     this._activedTrack = undefined
     this._viewMode = undefined
     this._viewOption = {}

modules/layer/Layer.js

    */
   _addOverlay(overlay) {
     if (
-      overlay &&
-      overlay.overlayEvent &&
+      overlay?.overlayEvent &&
       !this._cache.hasOwnProperty(overlay.overlayId)
     ) {
       this._cache[overlay.overlayId] = overlay
    */
   _removeOverlay(overlay) {
     if (
-      overlay &&
-      overlay.overlayEvent &&
+      overlay?.overlayEvent &&
       this._cache.hasOwnProperty(overlay.overlayId)
     ) {
       this._delegate && overlay.overlayEvent.fire(OverlayEventType.REMOVE, this)
    * @returns {Layer}
    */
   addTo(viewer) {
-    if (viewer && viewer.addLayer) {
+    if (viewer?.addLayer) {
       viewer.addLayer(this)
     }
     return this

modules/viewer/Viewer.js

    */
   _addLayerGroup(layerGroup) {
     if (
-      layerGroup &&
-      layerGroup.layerGroupEvent &&
+      layerGroup?.layerGroupEvent &&
       !Object(this._layerGroupCache).hasOwnProperty(layerGroup.id)
     ) {
       layerGroup.layerGroupEvent.fire(LayerGroupEventType.ADD, this)
    */
   _removeLayerGroup(layerGroup) {
     if (
-      layerGroup &&
-      layerGroup.layerGroupEvent &&
+      layerGroup?.layerGroupEvent &&
       Object(this._layerGroupCache).hasOwnProperty(layerGroup.id)
     ) {
       layerGroup.layerGroupEvent.fire(LayerGroupEventType.REMOVE, this)
    * @private
    */
   _addLayer(layer) {
-    if (layer && layer.layerEvent) {
+    if (layer?.layerEvent) {
       !this._layerCache[layer.type] && (this._layerCache[layer.type] = {})
       if (!Object(this._layerCache[layer.type]).hasOwnProperty(layer.id)) {
         layer.layerEvent.fire(LayerEventType.ADD, this)
    */
   _removeLayer(layer) {
     if (
-      layer &&
-      layer.layerEvent &&
+      layer?.layerEvent &&
       Object(this._layerCache[layer.type]).hasOwnProperty(layer.id)
     ) {
       layer.layerEvent.fire(LayerEventType.REMOVE, this)
    */
   hasLayer(layer) {
     return (
-      layer &&
-      layer.layerEvent &&
+      layer?.layerEvent &&
       Object(this._layerCache[layer.type]).hasOwnProperty(layer.id)
     )
   }