Error: `siteUrl` does not exist on `siteMetadata` in the data returned from the query. How to resolve?

When building my Gatsby website I get the below error. I've tried to delete and reinstall npm, update the plugins, deleting (Gatsby) cache, playing around with the siteUrl and all kinds of settings in the Gatsby config. But I can't seem to get rid of the error. The development environment works fine.

github: https://github.com/bartfluitsma/gatsby-bart-fluitsma

**Error console log**

ERROR #11321  PLUGIN

"gatsby-plugin-sitemap" threw an error while running the onPostBuild lifecycle:

`siteUrl` does not exist on `siteMetadata` in the data returned from the query.
Add this to your `siteMetadata` object inside gatsby-config.js or add this to your custom query or provide a custom `resolveSiteUrl` function.
https://www.gatsbyjs.com/plugins/gatsby-plugin-sitemap/#api-reference


  47 |             errors = _yield$graphql.errors;
  48 |             _context.next = 9;
> 49 |             return Promise.resolve(resolveSiteUrl(queryRecords)).catch(function (err) {
     |                                    ^
  50 |               return reporter.panic(_internals.REPORTER_PREFIX + " Error resolving Site URL", err);
  51 |             });
  52 |

File: node_modules\gatsby-plugin-sitemap\gatsby-node.js:49:36



  Error: `siteUrl` does not exist on `siteMetadata` in the data returned from the query.
  Add this to your `siteMetadata` object inside gatsby-config.js or add this to your custom query or provide a custom `resolveSiteUrl` function.
  https://www.gatsbyjs.com/plugins/gatsby-plugin-sitemap/#api-reference

  - internals.js:62 resolveSiteUrl
    [gatsby-bart-fluitsma]/[gatsby-plugin-sitemap]/internals.js:62:11

  - gatsby-node.js:49 _callee$
    [gatsby-bart-fluitsma]/[gatsby-plugin-sitemap]/gatsby-node.js:49:36


not finished onPostBuild - 0.480s

Gatsby-config.js

module.exports = {
  siteMetadata: {
    title: `Bart Fluitsma`,
    description: `Kick off your next, great Gatsby project with this default starter. This barebones starter ships with the main Gatsby configuration files you might need.`,
    author: `@gatsbyjs`,
    siteUrl: `http://bartfluitsma.com`,
  },
  plugins: [
    `gatsby-plugin-react-helmet`,
    `gatsby-plugin-image`,
    'gatsby-plugin-postcss',
    {
      resolve: `gatsby-plugin-google-fonts-with-attributes`,
      options: {
        fonts: [
          `montserrat\:300,400,400i,600,900`,
        ],
        display: 'swap',
        attributes: {
          rel: "stylesheet preload prefetch",
        },
      },
    },
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `images`,
        path: `${__dirname}/src/images`,
      },
    }, {
      resolve: `gatsby-source-filesystem`,
      options: {
        path: `${__dirname}/src/locales`,
        name: `locale`
      }
    },
    `gatsby-transformer-sharp`,
    `gatsby-plugin-sharp`,
    {
      resolve: `gatsby-plugin-manifest`,
      options: {
        name: `Web development | Bart Fluitsma`,
        short_name: `Bart develops`,
        start_url: `/`,
        background_color: `#663399`,
        // This will impact how browsers show your PWA/website
        // https://css-tricks.com/meta-theme-color-and-trickery/
        // theme_color: `#663399`,
        display: `minimal-ui`,
        icon: `src/images/logo-bart-fluitsma-web-design.svg`, // This path is relative to the root of the site.
      },
    },
    {
      resolve: `gatsby-plugin-react-i18next`,
      options: {
        localeJsonSourceName: `locale`, // name given to `gatsby-source-filesystem` plugin.
        languages: [`en`, `nl`],
        defaultLanguage: `en`,
        // if you are using Helmet, you must include siteUrl, and make sure you add http:https
        siteUrl: `https://bartfluitsma.com`,
        // you can pass any i18next options
        i18nextOptions: {
          interpolation: {
            escapeValue: false // not needed for react as it escapes by default
          },
          keySeparator: false,
          nsSeparator: false
        },
        pages: [
          {
            matchPath: '/:lang?/blog/:uid',
            getLanguageFromPath: true
          },
        ]
      }
    },
    // this (optional) plugin enables Progressive Web App + Offline functionality
    // To learn more, visit: https://gatsby.dev/offline
    // `gatsby-plugin-offline`,
    {
      resolve: 'gatsby-plugin-sitemap',
      options: {
        excludes: ['/**/404', '/**/404.html'],
        query: `
            {
              site {
                siteMetadata {
                  siteUrl
                }
              }
              allSitePage(filter: {context: {i18n: {routed: {eq: false}}}}) {
                edges {
                  node {
                    context {
                      i18n {
                        defaultLanguage
                        languages
                        originalPath
                      }
                    }
                    path
                  }
                }
              }
            }
          `,
        serialize: ({ site, allSitePage }) => {
          return allSitePage.edges.map((edge) => {
            const { languages, originalPath, defaultLanguage } = edge.node.context.i18n;
            const { siteUrl } = site.siteMetadata;
            const url = siteUrl + originalPath;
            const links = [
              { lang: defaultLanguage, url },
              { lang: 'x-default', url }
            ];
            languages.forEach((lang) => {
              if (lang === defaultLanguage) return;
              links.push({ lang, url: `${siteUrl}/${lang}${originalPath}` });
            });
            return {
              url,
              changefreq: 'daily',
              priority: originalPath === '/' ? 1.0 : 0.7,
              links
            };
          });
        }
      }
    },
  ],
}