Vue.jsのエラー vue.runtime.esm.js?2b0e:619 [Vue warn]: Property or method....

Vue ClIでの実装中発生したエラーの件なのですが
下記のようなエラーが出ており...

色城調査したのですが、解決に至らず...

エラー

vue.runtime.esm.js?2b0e:619 [Vue warn]: Property or method "loading" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

found in

---> <Loading> at src/components/Loading.vue
       <App> at src/App.vue
         <Root>

App.vue

<template>
  <div id="app">
    <Loading v-show="loading"></Loading>
    <Header></Header>
    <transition mode="out-in"> <!-- 「mode="out-in"」を追加する -->
      <router-view/>
    </transition>
    <Footer></Footer>
  </div>
</template>

<script>
import Loading from '@/components/Loading'
import Header from './components/Header.vue';
import Footer from './components/Footer.vue';

export default {
  name: 'App',
  data() {
    return {
      loading: true,
    }
  },
  components: {
    Loading,
    Header,
    Footer
  },
  mounted() {
    setTimeout(() => {
      this.loading = false;
    }, 1000);
  },
}
</script>

<style>

* {
  margin: 0;
  padding: 0;
}

ul, li {
  list-style-type: none;
}

img {
  max-width: 100%;
  height: auto;
}

a {
  text-decoration: none;
  color: #000;
}

a:hover {
  color: #000;
}
button {
  border: 0;
  background-color: transparent;
}
.v-enter-active, .v-leave-active {
  transition: opacity .5s;
}
.v-enter, .v-leave-to {
  opacity: 0;
}
</style>

main.js

import Vue from 'vue'
import App from './App.vue'
import axios from 'axios' //追記
import VueAxios from 'vue-axios' //追記
import router from './route.js'

import Headline from './components/Headline.vue'
import ItemBox from './components/ItemBox.vue'
import Menu from './components/Menu.vue'
import Top from './components/Top.vue'

Vue.config.productionTip = false

Vue.use(VueAxios, axios) //追記

Vue.component( 'Headline', Headline );
Vue.component( 'ItemBox', ItemBox );
Vue.component( 'Menu', Menu );
Vue.component( 'Top', Top );

new Vue({
  router,
  render: h => h(App),
}).$mount('#app')

よろしくお願いいたします。