don't crash when we don't have a subscription anymore

src/domain/ViewModel.js

@@ -25,6 +25,7 @@ export class ViewModel extends EventEmitter {
     constructor({clock, emitChange} = {}) {
         super();
         this.disposables = null;
+        this._isDisposed = false;
         this._options = {clock, emitChange};
     }
 
@@ -43,6 +44,11 @@ export class ViewModel extends EventEmitter {
         if (this.disposables) {
             this.disposables.dispose();
         }
+        this._isDisposed = true;
+    }
+
+    get isDisposed() {
+        return this._isDisposed;
     }
 
     disposeTracked(disposable) {

src/domain/session/room/timeline/TimelineViewModel.js

@@ -50,6 +50,10 @@ export class TimelineViewModel extends ViewModel {
      * @return {bool} startReached if the start of the timeline was reached
      */
     async loadAtTop() {
+        if (this.isDisposed) {
+            // stop loading more, we switched room
+            return true;
+        }
         const firstTile = this._tiles.getFirst();
         if (firstTile.shape === "gap") {
             return firstTile.fill();