graphql introspection returns null data

[audunhalland]

Using ra-data-opencrud (https://github.com/Weakky/ra-data-opencrud) which again uses introspection.js in ra-data-graphql:

What you were expecting:
I would expect introspection not to crash, because inspecting the network request shows that there is in fact a valid schema returned by the server.

What happened instead:
the graphql introspection query throws an error:

Uncaught (in promise) TypeError: Cannot read property '__schema' of null
    at introspection.js:76

Steps to reproduce:

1. Run a Prisma server which returns a valid response to introspection at localhost:4466
2. Create a simple react app using OpenCrud provider

Related code:

import React, { Component } from 'react';
import buildOpenCrudProvider from 'ra-data-opencrud';
class App extends Component {
  ...
  componentDidMount() {
    buildOpenCrudProvider({ clientOptions: { uri: 'http://localhost:4466/' } })
      .then(dataProvider => this.setState({ dataProvider }));
  }
  ...
}

Other information:
A bit of investigation from my end points in the direction of fetchPolicy in apollo client. React-admin already sets fetchPolicy: 'network-only' for ordinary graphql queries, (which I figure is a fix for this exact problem) but not for the introspection. A related apollo-client issue is apollographql/apollo-client#3030, but it appears it has never been properly fixed. I suggest setting fetchPolicy: 'network-only' to the introspection query also to work around this bug in apollo-client.

Environment

• React-admin version: 2.3.3
• Last version that did not exhibit the issue (if applicable):
• React version: 16.4.14
• Browser: any Chrome/Firefox
• Stack trace (in case of a JS error):

The crashing code in is in packages/ra-data-graphql/src/introspection.js:

export default async (client, options) => {
    const schema = options.schema
        ? options.schema
        : await client
              .query({
                  query: gql`
                      ${introspectionQuery}
                  `,
              })
              .then(({ data: { __schema } }) => __schema); // <-- crashes here: data is null
    ...
};