fix(plugin): always get global $nuxt while loading (#364)

Author: erraX

lib/plugin.js

@@ -2,8 +2,6 @@ import Axios from 'axios'
 import defu from 'defu'
 <% if (options.retry) { %>import axiosRetry from 'axios-retry'<% } %>
 
-const $nuxt = typeof window !== 'undefined' && window['$<%= options.globalName %>']
-
 // Axios.prototype cannot be modified
 const axiosExtra = {
   setBaseURL (baseURL) {
@@ -128,7 +126,10 @@ const setupProgress = (axios) => {
     set: () => { }
   }
 
-  const $loading = () => ($nuxt && $nuxt.$loading && $nuxt.$loading.set) ? $nuxt.$loading : noopLoading
+  const $loading = () => {
+    const $nuxt = typeof window !== 'undefined' && window['$<%= options.globalName %>']
+    return ($nuxt && $nuxt.$loading && $nuxt.$loading.set) ? $nuxt.$loading : noopLoading
+  }
 
   let currentRequests = 0