[Episode!]! represents an array of Episode objects. Since it is also non-nullable, you can always expect an array (with zero or more items) when you query the appearsIn field. And since Episode! is also non-nullable, you can always expect every item of the array to be an Episode object.
Note that this still allows an empty array, [].
It only disallows: null and [null].
But in GraphQL, all types are nullable by default, and you opt into non-nullability
In other typed languages, like TypeScript, Flow, Rust, or Haskell, all types are non-nullable by default, and you opt into nullability.
The exception can be avoided by using ? operator on the nullable value instead:
The @ ? annotation can be used to denote a nullable value.
this implies direct work with nil-able values which may end up with errors.
To get the final value you can use value_or which is a safe way to unwrap a nil-able value.