NODE_ENV not in default config causes error

[pstjean]

As of 3.2.2, when NODE_ENV is not set to one of the values in the default config (webpacker/lib/install/config/webpacker.yml) i.e. development, test or production, you get the following error:

  delete defaultConfig.extensions
         ^

TypeError: Cannot convert undefined or null to object
    at Object.<anonymous> (node_modules/@rails/webpacker/package/config.js:15:10)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (node_modules/@rails/webpacker/package/rules/babel.js:2:24)
    at Module._compile (module.js:570:32)

This was introduced in #1253, and it will only occur if there are extensions configured in the app configuration.

We discovered this when deploying to our stage environment.

[pstjean]

Closing as a duplicate of #1265