fix review & add test

packages/gatsby-plugin-sitemap/README.md

@@ -21,10 +21,6 @@ plugins: [`gatsby-plugin-sitemap`]
 Above is the minimal configuration required to have it work. By default, the
 generated sitemap will include all of your site's pages, except the ones you exclude.
 
-## Note
-
-`siteUrl` is required with `gatsby-plugin-sitemap`.
-
 ## Options
 
 The `defaultOptions` [here](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sitemap/src/internals.js#L34) can be overridden.

packages/gatsby-plugin-sitemap/src/__tests__/internals.js

@@ -69,6 +69,20 @@ describe(`results using default settings`, () => {
 
       verifyUrlsExistInResults(urls, [`http://dummy.url${pathPrefix}/page-1`])
     })
+
+    it(`should fail when siteUrl is not set`, async () => {
+      const graphql = () =>
+        Promise.resolve(generateQueryResultsMock({ siteUrl: null }))
+      expect.assertions(1)
+
+      try {
+        await runQuery(graphql, ``, [], pathPrefix)
+      } catch (err) {
+        expect(err.message).toEqual(
+          expect.stringContaining(`SiteMetaData 'siteUrl' property is required`)
+        )
+      }
+    })
   }
 
   describe(`no path-prefix`, () => {

packages/gatsby-plugin-sitemap/src/internals.js

@@ -31,7 +31,7 @@ export const runQuery = (handler, query, excludes, pathPrefix) =>
 
     if (!r.data.site.siteMetadata.siteUrl) {
       throw new Error(
-        `SiteMetaData 'siteUrl' property is required. (https://www.gatsbyjs.org/packages/gatsby-plugin-sitemap/#how-to-use)`
+        `SiteMetaData 'siteUrl' property is required. Check out the documentation to see a working example: https://www.gatsbyjs.org/packages/gatsby-plugin-sitemap/#how-to-use`
       )
     }