title: Vuex $state issue
body: <p>I have error like this:
  I previously thought that this is problem with router, but after creating new project with only vuex problem is still there</p>
  
  <p><a href="https://i.stack.imgur.com/YIPp5.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>This is my main.js</p>
  
  <pre><code>import Vue from 'vue'
  import App from './App'
  import store from './store'
  
  new Vue({
    el: "#app",
    store,
    render: h =&gt; h(App)
  })
  </code></pre>
  
  <p>store.js:</p>
  
  <pre><code>import Vue from 'vue'
  import Vuex from 'vuex'
  
  Vue.use(Vuex)
  
  export const store = new Vuex.Store({
    state: {
      users: [
        {id: 1, name: 'Adrian', email: 'adrian@email.pl'},
        {id: 2, name: 'Magda', email: 'magda@email.pl'}
      ]
    }
  })
  </code></pre>
  
  <p>and my component using $store:</p>
  
  <pre><code>&lt;template&gt;
    &lt;div class="usersNames"&gt;
      &lt;ul&gt;
        &lt;li v-for="user in usersNames"&gt;{{user.name}}&lt;/li&gt;
      &lt;/ul&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
    export default {
      name: 'usersNames',
      computed: {
        usersNames() {
          return this.$store.state.users;
        }
      }
    }
  &lt;/script&gt;
  </code></pre>