Fix return type on funcs in matrixClient to be optionally null

src/client.ts

@@ -1403,7 +1403,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      *
      * @return {?string} MXID for the logged-in user, or null if not logged in
      */
-    public getUserId(): string {
+    public getUserId(): string | null {
         if (this.credentials && this.credentials.userId) {
             return this.credentials.userId;
         }
@@ -1425,7 +1425,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * Get the local part of the current user ID e.g. "foo" in "@foo:bar".
      * @return {?string} The user ID localpart or null.
      */
-    public getUserIdLocalpart(): string {
+    public getUserIdLocalpart(): string | null {
         if (this.credentials && this.credentials.userId) {
             return this.credentials.userId.split(":")[0].substring(1);
         }
@@ -1480,7 +1480,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @param {string} roomId The room the call is to be placed in.
      * @return {MatrixCall} the call or null if the browser doesn't support calling.
      */
-    public createCall(roomId: string): MatrixCall {
+    public createCall(roomId: string): MatrixCall | null {
         return createNewMatrixCall(this, roomId);
     }
 
@@ -1788,7 +1788,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      *
      * @return {module:crypto/deviceinfo} device or null
      */
-    public getStoredDevice(userId: string, deviceId: string): DeviceInfo {
+    public getStoredDevice(userId: string, deviceId: string): DeviceInfo | null {
         if (!this.crypto) {
             throw new Error("End-to-end encryption disabled");
         }
@@ -3313,7 +3313,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @return {?User} A user or null if there is no data store or the user does
      * not exist.
      */
-    public getUser(userId: string): User {
+    public getUser(userId: string): User | null {
         return this.store.getUser(userId);
     }
 
@@ -6806,7 +6806,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * Get the access token associated with this account.
      * @return {?String} The access_token or null
      */
-    public getAccessToken(): string {
+    public getAccessToken(): string | null {
         return this.http.opts.accessToken || null;
     }
 
@@ -8898,7 +8898,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @param {string} roomId The room ID to get a tree space reference for.
      * @returns {MSC3089TreeSpace} The tree space, or null if not a tree space.
      */
-    public unstableGetFileTreeSpace(roomId: string): MSC3089TreeSpace {
+    public unstableGetFileTreeSpace(roomId: string): MSC3089TreeSpace | null {
         const room = this.getRoom(roomId);
         if (room?.getMyMembership() !== 'join') return null;