Appearance settings tab crashes when custom FE is served by backend

Environment info

• Browser, version, OS, platform: Chromium and Firefox
• Instance URL: N/A
• Frontend version (see settings -> about): develop
• Backend version (see settings -> about): stable
• Browser extensions (ublock, rikaichamp etc): N/A (Chrome incognito window)
• Known instance/user customizations (i.e. pleromafe mods/forks, instance styles etc): FE served from $instance_static/frontends/fe/dist

Bug description & reproduction steps

Appearance settings tab crashes almost immediately when custom FE is served by BE. Running FE in dev mode with yarn dev and proxied to the local BE instance works without issues.

Steps to reproduce:

1. Configure BE to serve a different primary frontend with config :pleroma, :frontends, primary: %{"name" => "fe", "ref" => "dist"}
2. Build a prod version of the FE with yarn build
3. Copy dist folder to $PLEROMA_BE/instance/static/frontends/fe
4. Start BE and go to localhost:4000
5. Open settings and switch to the appearance tab
6. The tab disappears almost immediately

Last known good commit: 898881ba

Console output in Chromium devtools:

Bug seriousness

• How annoying it is: Minor
• How often does it happen: Every time
• How many people does it affect: Not many