message: Better null check
  to make tests happy
diff: @@ -118,7 +118,7 @@ function getRemoteAudioElement(): HTMLAudioElement {
  export default class CallHandler {
  private calls = new Map<string, MatrixCall>(); // roomId -> call
  private audioPromises = new Map<AudioID, Promise<void>>();
  - private dispatcherRef: string;
  + private dispatcherRef: string = null;
  static sharedInstance() {
  if (!window.mxCallHandler) {
  @@ -152,7 +152,7 @@ export default class CallHandler {
  if (cli) {
  cli.removeListener('Call.incoming', this.onCallIncoming);
  }
  - if (this.dispatcherRef) dis.unregister(this.dispatcherRef);
  + if (this.dispatcherRef !== null) dis.unregister(this.dispatcherRef);
  }
  private onCallIncoming = (call) => {
  