Improve quality of Typescript types (#10742)

src/stores/ActiveWidgetStore.ts

@@ -39,8 +39,8 @@ export enum ActiveWidgetStoreEvent {
  */
 export default class ActiveWidgetStore extends EventEmitter {
     private static internalInstance: ActiveWidgetStore;
-    private persistentWidgetId: string | null;
-    private persistentRoomId: string | null;
+    private persistentWidgetId: string | null = null;
+    private persistentRoomId: string | null = null;
     private dockedWidgetsByUid = new Map<string, number>();
 
     public static get instance(): ActiveWidgetStore {

src/stores/OwnProfileStore.ts

@@ -43,7 +43,7 @@ export class OwnProfileStore extends AsyncStoreWithClient<IState> {
         return instance;
     })();
 
-    private monitoredUser: User | null;
+    private monitoredUser: User | null = null;
 
     private constructor() {
         // seed from localstorage because otherwise we won't get these values until a whole network

src/stores/local-echo/GenericEchoChamber.ts

@@ -28,7 +28,7 @@ export const PROPERTY_UPDATED = "property_updated";
 
 export abstract class GenericEchoChamber<C extends EchoContext, K, V> extends EventEmitter {
     private cache = new Map<K, { txn: EchoTransaction; val: V }>();
-    protected matrixClient: MatrixClient | null;
+    protected matrixClient: MatrixClient | null = null;
 
     protected constructor(public readonly context: C, private lookupFn: (key: K) => V) {
         super();