Getting a js error when going to /graphql/explorer

When I go to /graphql/explorer - I get the following javascript error:

Uncaught TypeError: Cannot read property '__schema' of undefined
at i (bundle.min.js?v=8.7.8:1)
at Object.attach (bundle.min.js?v=8.7.8:1)
at drupal.js?v=8.7.8:25
at Array.forEach ()
at Object.Drupal.attachBehaviors (drupal.js?v=8.7.8:22)
at drupal.init.js?v=8.7.8:16
at HTMLDocument.t (ready.min.js?v=1.0.8:4)

I've installed Drupal 8.7.8, the Demo: Umami Food Magazine (Experimental) (demo_umami-8.7.8) profile, the webonyx/graphql-php component, GRAPHQL and GRAPHQL Core both 8.x-3.0-rc3.

Any ideas on what might be wrong/missing?

Do I need to do something to create schemas? If I understand correctly, the schemas will get generated automatically. Is that right? Is there a way for me to check if there are schemas created.