Destroy view-models when destroying trees

app/src/model/TopicViewModel.ts

@@ -1,6 +1,7 @@
+import { Destroyable } from '../../../backend/src/Model/Destroyable'
 import { EventDispatcher } from '../../../events'
 
-export class TopicViewModel {
+export class TopicViewModel implements Destroyable {
   private selected: boolean
   public change = new EventDispatcher<void, TopicViewModel>()
 
@@ -8,6 +9,10 @@ export class TopicViewModel {
     this.selected = false
   }
 
+  public destroy() {
+    this.change.removeAllListeners()
+  }
+
   public isSelected() {
     return this.selected
   }