message: fix: add null check in dispose method
diff: -import { observe } from 'mobx';
  +import { observe, Lambda } from 'mobx';
  import { OpenAPISpec } from '../types';
  import { loadAndBundleSpec } from '../utils/loadAndBundleSpec';
  @@ -58,7 +58,7 @@ export class AppStore {
  marker = new MarkerService();
  private scroll: ScrollService;
  - private disposer;
  + private disposer: Lambda | null = null;
  constructor(
  spec: OpenAPISpec,
  @@ -96,8 +96,10 @@ export class AppStore {
  dispose() {
  this.scroll.dispose();
  this.menu.dispose();
  + if (this.disposer != null) {
  this.disposer();
  }
  + }
  /**
  * serializes store
  