fix detail page empty if the monitor list is not yet ready

src/pages/Details.vue

@@ -1,6 +1,6 @@
 <template>
     <transition name="slide-fade" appear>
-        <div>
+        <div v-if="monitor">
             <h1> {{ monitor.name }}</h1>
             <p class="url">
                 <a v-if="monitor.type === 'http' || monitor.type === 'keyword' " :href="monitor.url" target="_blank">{{ monitor.url }}</a>