title: Cannot resolve symbol 'AppCompatActivity' -- NO SOLUTION
body: <p>I am currently developing an android application.
  Everything has been working fine, up until the point I decided to add my project to firebase.
  I did the steps <code>Tools &gt; Firebase &gt; Authentication</code>.
  I created the project and added firebase to my application.</p>
  <p>5 seconds later, I get the <code>Cannot resolve symbol 'AppCompatActivity' error</code>.</p>
  <p>I know there are lots of other threads on this issue, and I have read them.</p>
  <p>However, no solution seems to work for me. I have tried:</p>
  <pre><code>&gt;- Syncing project with gradle files.
  &gt;- Change the implementation version in app.gradle
  &gt;- Invalidate caches and Restart
  &gt;- Delete .idea folder in project path
  &gt;- Uninstall and then reinstall Android Studio
  </code></pre>
  <p>All of these with no success.
  Is there anything else I can do about this? Why does this error occur anyway? Did I mess something?</p>