Demonstrates an issue I currently have in Gatsby 3

Reproduce:

git clone https://github.com/CodeAndWeb/gatsby-3-migration-issue.git
yarn install

Everything is ok from the start:

gatsby develop --verbose

Visit http://localhost:8000/___graphql

Edit gatsby-node.js and uncomment the following lines:

const result = await graphql(`
query {
      allMdx {
        nodes {
          slug
        }
      }
}
`)
console.log(JSON.stringify(result, null, 4))

Re-start gatsby and visit http://localhost:8000/___graphql