title: An element descriptor's .kind property must be either "method" or "field"
body: <p>I'm following the documentation for <a href="https://github.com/alisd23/mobx-react-router" rel="nofollow noreferrer"><code>mobx-react-router</code></a> but upon attempting to run my application I get the following error in the browser:</p>
  
  <pre><code>Uncaught TypeError: An element descriptor's .kind property must be either "method" or "field", but a decorator created an element descriptor with .kind "undefined"
      at _toElementDescriptor (app.js:49988)
      at _toElementFinisherExtras (app.js:49990)
      at _decorateElement (app.js:49980)
      at app.js:49976
      at Array.forEach (&lt;anonymous&gt;)
      at _decorateClass (app.js:49976)
      at _decorate (app.js:49958)
      at Module../src/App/UserStore.js (app.js:50012)
      at __webpack_require__ (bootstrap:19)
      at Module../src/index.js (index.js:1)
  </code></pre>
  
  <p>Here is how I intitialize: </p>
  
  <pre><code>const appContainer = document.getElementById('app');
  if(appContainer) {
    const browserHistory = createBrowserHistory()
    const routingStore = new RouterStore();
  
    const stores = {
      users: userStore,
      routing: routingStore
    }
  
    const history = syncHistoryWithStore(browserHistory, routingStore);
  
    ReactDOM.render(
      (
        &lt;Provider {...stores}&gt;
          &lt;Router history={history}&gt;
            &lt; App /&gt;
          &lt;/Router&gt;
        &lt;/Provider&gt;
      ),
    appContainer);
  }
  </code></pre>
  
  <p>And this is how I use:</p>
  
  <pre><code>@inject('routing')
  @inject('users')
  @observer
  class App extends Component { ...
  </code></pre>
  
  <p>My <code>UserStore</code>:</p>
  
  <pre><code>import { observable, action, computed } from "mobx"
  
  class UserStore {
    @observable users = [];
  
    @action addUser = (user) =&gt; {
      this.users.push(user)
    }
  
    @computed get userCount () {
      return this.users.length
    }
  }
  
  const store = new UserStore();
  export default store;
  </code></pre>
  
  <p>I've tried to Google for this error but it's returning no useful results. Any ideas what I am doing wrong?</p>