SSR: `Cannot read property 'get' of undefined` [cssModulesCache] since v2.3.4

Describe the bug

After updating to v2.3.4, I get this error when trying to render a Svelte file with CSS in development mode:

TypeError: Cannot read property 'get' of undefined
    at TransformContext.transform (/my-project/node_modules/vite/src/node/plugins/css.ts:256:51)
    at Object.transform (/my-project/node_modules/vite/src/node/server/pluginContainer.ts:495:43)
    at transformRequest (/my-project/node_modules/vite/src/node/server/transformRequest.ts:128:27)
    at instantiateModule (/my-project/node_modules/vite/src/node/ssr/ssrModuleLoader.ts:68:6) {
  plugin: 'vite:css-post',
  id: '/my-project/src/components/SettingsSidebar.svelte?svelte&type=style&lang.css',
  pluginCode: '.SidebarNav.s-6BsyqWO9S_5d{color:black}.s-6BsyqWO9S_5d{}'
}

The error points to this line: https://github.com/vitejs/vite/blob/b6d12f71c1dbd5562f25bc2c32c44eed32b27e94/packages/vite/src/node/plugins/css.ts, so the undefined of the error message probably refers to the cssModulesCache.

The line has recently been changed for this PR: https://github.com/vitejs/vite/pull/3516/files

Reproduction

I hope that this change was recent enough and the problem is obvious enough that I don’t have to assemble a reproduction. If you still need one, tell me!

System Info

Output of npx envinfo --system --npmPackages vite,@vitejs/plugin-vue --binaries --browsers:

  System:
    OS: macOS 11.3.1
    CPU: (16) x64 Intel(R) Xeon(R) W-2140B CPU @ 3.20GHz
    Memory: 805.53 MB / 32.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.16.1 - ~/.nvm/versions/node/v14.16.1/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.12 - ~/.nvm/versions/node/v14.16.1/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Browsers:
    Chrome: 90.0.4430.212
    Edge: 90.0.818.66
    Firefox: 87.0
    Firefox Developer Edition: 89.0
    Safari: 14.1
    Safari Technology Preview: 14.2
  npmPackages:
    vite: ^2.3.4 => 2.3.4

Used package manager: npm

[danieldiekmeier]

@AlonMiz Thanks for the hint, that helped me to fix it in my app!

My problem was that I had this code on the server for SSR:

const viteServer = createServer()

This used to work, but since Vite v2.3.4, I got the mentioned error.

But if I explicitely set the middlewareMode, it works again:

const viteServer = createServer({ server: { middlewareMode: 'ssr' } })

I currently don’t have time to provide a more complete reproduction than that, sorry. 😦 In any case, I can at least work around the error for now and continue with my app. 🚀

[github-actions[bot]]

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