MOBILE-4081 addons: Delete manager instances

This prevents errors for trying to get the source of a destroyed manager (for example, for subsequent asynchronous calls fetching data).

src/addons/calendar/components/calendar/calendar.ts

@@ -340,7 +340,9 @@ export class AddonCalendarCalendarComponent implements OnInit, DoCheck, OnDestro
     ngOnDestroy(): void {
         this.undeleteEventObserver?.off();
         this.manager?.destroy();
-        this.managerUnsubscribe && this.managerUnsubscribe();
+        this.managerUnsubscribe?.();
+
+        delete this.manager;
     }
 
 }

src/addons/calendar/pages/day/day.page.ts

@@ -457,7 +457,9 @@ export class AddonCalendarDayPage implements OnInit, OnDestroy {
         this.filterChangedObserver?.off();
         this.manager?.getSource().forgetRelatedSources();
         this.manager?.destroy();
-        this.managerUnsubscribe && this.managerUnsubscribe();
+        this.managerUnsubscribe?.();
+
+        delete this.manager;
     }
 
 }

src/addons/mod/book/pages/contents/contents.ts

@@ -314,8 +314,10 @@ export class AddonModBookContentsPage implements OnInit, OnDestroy {
      * @inheritdoc
      */
     ngOnDestroy(): void {
-        this.managerUnsubscribe && this.managerUnsubscribe();
         this.manager?.destroy();
+        this.managerUnsubscribe?.();
+
+        delete this.manager;
     }
 
 }