Handle Redux store subscriptions on WidgetController destruction

h/static/scripts/widget-controller.js

@@ -93,7 +93,7 @@ module.exports = function WidgetController(
   // only those threads, using placeholders above and below the visible threads
   // to reserve space for threads which are not actually rendered.
   var visibleThreads = new VirtualThreadList($scope, window, thread());
-  annotationUI.subscribe(function () {
+  var unsubscribeAnnotationUI = annotationUI.subscribe(function () {
     visibleThreads.setRootThread(thread());
     $scope.selectedTab = annotationUI.getState().selectedTab;
 
@@ -101,6 +101,8 @@ module.exports = function WidgetController(
     $scope.totalNotes = countNotes(annotationUI.getState().annotations);
   });
 
+  $scope.$on('$destroy', unsubscribeAnnotationUI);
+
   visibleThreads.on('changed', function (state) {
     $scope.virtualThreadList = {
       visibleThreads: state.visibleThreads,