title: swagger error for my upgraded .netcore app
body: <p>So I do have this web api built in .netcore 2.2 when Im on the process of upgrading this error shows up</p>
  <p>System.InvalidOperationException: 'API Explorer not registered in DI.'</p>
  <p>I already upgraded my swagger nugget and follow some instructions in upgrading but this error still pops up.</p>
  <p><a href="https://i.stack.imgur.com/OREjL.png" rel="nofollow noreferrer">Click for the image of the error</a></p>