Default layouts are not working with `gatsby-theme-mdx-deck`

Default layouts are not working with MDX Deck.

Example repo

// gatsby-config.js
module.exports = {
  plugins: [
    {
      resolve: 'gatsby-theme-mdx-deck',
      options: {
        contentPath: './src/decks',
        basePath: '/',
      }
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `decks`,
        path: `${__dirname}/src/decks/`,
      },
    },
    {
      resolve: `gatsby-plugin-page-creator`,
      options: {
        path: `${__dirname}/src/decks`,
      },
    },
    {
      resolve: `gatsby-plugin-mdx`,
      options: {
        defaultLayouts: {
          default: require.resolve(`./src/components/Layout.js`),
        }
      }
    }
  ]
}

// Layout.js
import React from 'react'
import HeaderButton from './HeaderButton'

export default ({ children }) => (
  <>
    <HeaderButton />

    { children }
  </>
)

Syntax Error: SyntaxError: unknown: Identifier '_frontmatter' has already been declared (111:13)

  109 | MDXContent.isMDXComponent = true;
  110 |
> 111 | export const _frontmatter = {};
      |              ^
  112 |