Add method to await space store setup

Otherwise, the room list store will get incorrect information about
spaces and thus will produce an incorrect roomlist.

src/stores/spaces/SpaceStore.ts

@@ -21,6 +21,7 @@ import {
 } from "matrix-js-sdk/src/matrix";
 import { KnownMembership } from "matrix-js-sdk/src/types";
 import { logger } from "matrix-js-sdk/src/logger";
+import { defer } from "matrix-js-sdk/src/utils";
 
 import { AsyncStoreWithClient } from "../AsyncStoreWithClient";
 import defaultDispatcher from "../../dispatcher/dispatcher";
@@ -152,6 +153,7 @@ export class SpaceStoreClass extends AsyncStoreWithClient<EmptyObject> {
     private _enabledMetaSpaces: MetaSpace[] = [];
     /** Whether the feature flag is set for MSC3946 */
     private _msc3946ProcessDynamicPredecessor: boolean = SettingsStore.getValue("feature_dynamic_room_predecessors");
+    private _isReady = defer();
 
     public constructor() {
         super(defaultDispatcher, {});
@@ -162,6 +164,14 @@ export class SpaceStoreClass extends AsyncStoreWithClient<EmptyObject> {
         SettingsStore.monitorSetting("feature_dynamic_room_predecessors", null);
     }
 
+    /**
+     * Returns a promise that resolves when the space store is ready.
+     * This happens after an initial hierarchy of spaces and rooms has been computed.
+     */
+    public get isReady(): Promise<void> {
+        return this._isReady.promise;
+    }
+
     /**
      * Get the order of meta spaces to display in the space panel.
      *
@@ -1201,6 +1211,7 @@ export class SpaceStoreClass extends AsyncStoreWithClient<EmptyObject> {
         } else {
             this.switchSpaceIfNeeded();
         }
+        this._isReady.resolve();
     }
 
     private sendUserProperties(): void {