title: Puppeteer waitForSelector not working as expected
body: <p>I've this simple code chunk:</p>
  <pre><code>const BUSINESS = 'lalala';
  await page.waitForSelector('#searchboxinput').then(
      page.evaluate( (BUSINESS) =&gt; {
          document.querySelector('#searchboxinput').value = BUSINESS
      }, BUSINESS)
   ),
  </code></pre>
  <p>If I set wait for selector -&gt; then, I would expect <strong>then</strong> to be executed <strong>when the selector exists</strong>, but I'm getting a <strong>Cannot set property value of 'null'</strong>.</p>
  <p>So I uderstand that document.querySelector('#searchboxinput') == undefined while I suppose that it cannot be possible as it's executed when waitForSelector promise is finished...</p>
  <p>is that a bug or I'm missing something?</p>