"TypeError: Cannot read property 'replace' of undefined" when docusaurus build

## 🐛 Bug Report

I am using Docusaurus 2.

I get this error when I run `docusaurus build`,
```
Creating an optimized production build...

✔ Client
  Compiled successfully in 1.21m

✖ Server
  Compiled with some errors in 1.35m

TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)TypeError: Cannot read property 'replace' of undefined
    at String.replace (<anonymous>)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
(undefined) TypeError: Cannot read property 'replace' of undefined
    at Object.options.minifyJS (main:18886:28)
    at Object.chars (main:19374:24)
    at main:48346:19
    at String.replace (<anonymous>)
    at new HTMLParser (main:48338:19)
    at minify (main:19160:3)
    at module.exports.exports.minify (main:19521:16)
    at serverEntry_render (main:88195:38)
Error: Failed to compile with errors.
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/@docusaurus/core/lib/webpack/utils.js:164:24
    at finalCallback (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/MultiCompiler.js:254:12)
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/MultiCompiler.js:277:6
    at done (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/neo-async/async.js:2931:13)
    at runCompilers (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/MultiCompiler.js:181:48)
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/MultiCompiler.js:188:7
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/MultiCompiler.js:270:7
    at finalCallback (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/Compiler.js:257:39)
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/Compiler.js:273:13
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:42:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/tapable/lib/Hook.js:154:20)
    at onCompiled (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/Compiler.js:271:21)
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/Compiler.js:681:15
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:6:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/tapable/lib/Hook.js:154:20)
    at /Users/kumarabhirup/Documents/Repositories/propagateAt/node_modules/webpack/lib/Compiler.js:678:31
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Yes

## To Reproduce

1. Use the following `docusaurus.config.js`
```
module.exports = {
  title: 'propagate.at',
  tagline: 'Email, WhatsApp and SMS Marketing for Online Creators',
  url: 'https://kb.propagate.at',
  baseUrl: '/',
  onBrokenLinks: 'throw',
  favicon: 'img/favicon.ico',
  organizationName: 'propagate-at', // Usually your GitHub org/user name.
  projectName: 'propagateAt', // Usually your repo name.
  themeConfig: {
    colorMode: {
      disableSwitch: true,
    },
    navbar: {
      // title: 'propagate.at',
      logo: {
        alt: 'PropagateAt Logo',
        src: 'img/PropagateAt Logo Light.svg'
      },
      items: [
        {
          to: 'docs/',
          activeBasePath: 'docs',
          label: 'Docs',
          position: 'left',
        },
        {to: 'blog', label: 'Press', position: 'left'},
        {
          href: 'https://github.com/propagate-at',
          label: 'GitHub',
          position: 'right',
        },
        {
          href: 'https://propagate.at/dashboard',
          label: 'Dashboard',
          position: 'right',
        },
      ],
    },
    footer: {
      style: 'light',
      logo: {
        alt: 'PropagateAt Logo',
        src: 'img/PropagateAt Logo Light.svg',
      },
      links: [
        {
          title: 'Docs',
          items: [
            {
              label: 'Developer API',
              to: 'docs/api/',
            },
            {
              label: 'Guides',
              to: 'docs/guides/',
            },
          ],
        },
        {
          title: 'Community',
          items: [
            {
              label: 'Discord',
              href: 'https://discord.gg/RHZMhgF',
            },
            {
              label: 'Twitter',
              href: 'https://twitter.com/propagateHQ',
            },
          ],
        },
        {
          title: 'Social',
          items: [
            // {
            //   label: 'Twitter',
            //   href: 'https://twitter.com/propagateHQ',
            // },
            {
              label: 'Facebook',
              href: 'https://fb.com/propagate.at',
            },
            {
              label: 'Instagram',
              href: 'https://www.instagram.com/propagate.at/',
            }
          ],
        },
        {
          title: 'More',
          items: [
            {
              label: 'Press',
              to: 'blog',
            },
            {
              label: 'GitHub',
              href: 'https://github.com/propagate-at',
            },
            {
              label: 'LinkedIn',
              href: 'https://www.linkedin.com/company/propagateAt',
            }
          ],
        },
      ],
      copyright: `Copyright © ${new Date().getFullYear()} PropagateAt, Inc. Built with ❤️ for creators.`,
    },
  },
  presets: [
    [
      '@docusaurus/preset-classic',
      {
        docs: {
          sidebarPath: require.resolve('./sidebars.js'),
          // Please change this to your repo.
          // editUrl:
          //   'https://github.com/facebook/docusaurus/edit/master/website/',
        },
        blog: {
          showReadingTime: true,
          // Please change this to your repo.
          // editUrl:
          //   'https://github.com/facebook/docusaurus/edit/master/website/blog/',
        },
        theme: {
          customCss: require.resolve('./src/css/custom.css'),
        },
      },
    ],
  ],
};
```

2. Run `docusaurus build`.

## Behavior

`docusaurus start` works perfectly fine for development purposes. After I got the expected result on dev stage, I ran `docusaurus build`. Then I got that error.

Btw, I am new to Docusaurus.

## Your Environment

- Docusaurus version used: 
```
"@docusaurus/core": "2.0.0-alpha.66",
"@docusaurus/preset-classic": "2.0.0-alpha.66"
```
- Environment name and version (e.g. Chrome 78.0.3904.108, Node.js 10.17.0): Chrome 86 | Node.js v15.0.1
- Operating system and version (desktop or mobile): MacOS Mojave

## Reproducible Demo

I will soon create a Demo Repo exclusively for this issue. Would like to get some time.

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"TypeError: Cannot read property 'replace' of undefined" when docusaurus build #3657

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"TypeError: Cannot read property 'replace' of undefined" when docusaurus build #3657

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Have you read the Contributing Guidelines on issues?

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