title: "Cannot read property 'upgrade' of undefined" when starting vue application
body: <p>I have a project that's been running perfectly for a few months now, able to run <code>npm run serve</code> with no problem. I have to terminate and run the command again whenever I switch networks, but that has never been a problem and the server could always start again.</p>
  
  <p>Until now - no matter what I do I can't get the server to start. I get this error:</p>
  
  <pre><code>npm run serve
  
  &gt; xbs@0.6.2 serve D:\workspace\[PROJECT]
  &gt; vue-cli-service serve
  
   INFO  Starting development server...
   10% building 1/1 modules 0 active ERROR  TypeError: Cannot read property 'upgrade' of undefined
  TypeError: Cannot read property 'upgrade' of undefined
      at Server.&lt;anonymous&gt; (D:\workspace\[PROJECT]\node_modules\webpack-dev-server\lib\Server.js:667:47)
      at Array.forEach (&lt;anonymous&gt;)
      at new Server (D:\workspace\[PROJECT]\node_modules\webpack-dev-server\lib\Server.js:666:22)
      at serve (D:\workspace\[PROJECT]\node_modules\@vue\cli-service\lib\commands\serve.js:137:20)
      at &lt;anonymous&gt;
      at process._tickCallback (internal/process/next_tick.js:188:7)
      at Function.Module.runMain (module.js:695:11)
      at startup (bootstrap_node.js:191:16)
      at bootstrap_node.js:612:3
  </code></pre>
  
  <p>The last time I got this message was when I forgot to set the <code>.env</code> files, but this is a previously working copy with all necessary files set, and <strong>no changes since the last working run</strong>.</p>
  
  <p>I've even tried pulling a fresh copy of the repo, running <code>npm i</code> and setting all env files but it still fails.</p>
  
  <p><code>npm run lint</code> and <code>npm run build</code> can still work, only <code>npm run serve</code>.</p>
  
  <p>Am I missing anything?</p>