@angular-builders/custom-webpack:karma fails compiling TypeScript with activated strictNullChecks

Describe the Bug

When a custom webpack config is provided written in TypeScript, @angular-builders/custom-webpack:karma throws an error if strictNullChecks are activated.

Minimal Reproduction

https://github.com/Markus-Ende/custom-webpack-builder-strictnullchecks-issue

1. Clone & install repo
2. run npm test.

(Deactive strictNullChecks in tsconfig.json to see, that the Error does only occur when activated)

Expected Behavior

Karma tests should run normally.

Error output

An unhandled exception occurred: ⨯ Unable to compile TypeScript:
webpack-config/extra-webpack.config.ts:2:54 - error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Function'.

2 export default (config) => __awaiter(void 0, void 0, void 0, function* () {
                                                       ~~~~~~

See "/tmp/ng-Hi3EUl/angular-errors.log" for further details.

Environment

Libs
- @angular/core version: 9.1.9
- @angular-devkit/build-angular version: 0.901.7
- @angular-builders/custom-webpack version: 9.1.0

For Tooling issues:
- Node version: v12.16.1
- Platform:  Linux

[just-jeb]

The issue is that there are two instances of ts-node registered during the run. No out of the box solution at the moment. As a first step I’d try to remove your direct ts-node dependency and if that doesn’t help I’d just use Promise.then syntax.

[just-jeb]

Ok. A few findings:

1. The problem is with async keyword - once it is removed the problem goes away. So as a workaround you can just use plain Promise syntax.
2. I can disable the type checking for the config file by passing transpileOnly option into register. Not sure it’s the right decision though (it is basically your only indication for type errors in your config).
3. Another solution would be to have a separate tsconfig.json (built in the builder) for config transpilation. Not sure this is the right decision either, since I do want people to have control of it.
4. I’d probably go with #2 or #3 if it was happening with all the commands. But it only happens with Karma and I don’t have any idea why. So I think it’s important to understand how Karma flow is different from the others before we take any actions.