As `npm run build` concludes, the `index` file is not sent to the renderer

When I run `npm build`, I get the following result in my terminal:

```bash
npm run build

> build
> concurrently "npm run build:main" "npm run build:renderer"

[1]
[1] > build:renderer
[1] > cross-env NODE_ENV=production TS_NODE_TRANSPILE_ONLY=true webpack --config ./.erb/configs/webpack.config.renderer.prod.ts
[1]
[0]
[0] > build:main
[0] > cross-env NODE_ENV=production TS_NODE_TRANSPILE_ONLY=true webpack --config ./.erb/configs/webpack.config.main.prod.ts && electron .
[0]
[0]
[0] 10:34:22.720 > Skip checkForUpdates because application is not packed and dev update config is not forced
[0] 10:34:22.729 > checkForUpdatesAndNotify called, downloadPromise is null
[0] (node:12984) electron: Failed to load URL: http://localhost:1212/index.html with error: ERR_CONNECTION_REFUSED
[0] (Use `electron --trace-warnings ...` to show where the warning was created)
[0] [12984:0311/103426.720:ERROR:CONSOLE(1)] "Request Autofill.enable failed. {"code":-32601,"message":"'Autofill.enable' wasn't found"}", source: devtools://devtools/bundled/core/protocol_client/protocol_client.js (1)
[0] [12984:0311/103426.721:ERROR:CONSOLE(1)] "Request Autofill.setAddresses failed. {"code":-32601,"message":"'Autofill.setAddresses' wasn't found"}", source: devtools://devtools/bundled/core/protocol_client/protocol_client.js (1)
[0] [12984:0311/103450.571:ERROR:CONSOLE(1)] "Request Autofill.enable failed. {"code":-32601,"message":"'Autofill.enable' wasn't found"}", source: devtools://devtools/bundled/core/protocol_client/protocol_client.js (1)
[0] [12984:0311/103450.571:ERROR:CONSOLE(1)] "Request Autofill.setAddresses failed. {"code":-32601,"message":"'Autofill.setAddresses' wasn't found"}", source: devtools://devtools/bundled/core/protocol_client/protocol_client.js (1)
[1] npm run build:renderer exited with code 0
[0] npm run build:main exited with code 0
```

The build then *successfully* finishes and spits out a blank screen, like so:


I have uploaded the repro version, called [react-colours-legacy](https://github.com/LazarPetrovic1/react-colours-legacy).