`yarn dev` says "path argument must be of type string" about svelte-webpack-starter HOT 4 CLOSED

#6 seems to fix this issue on my end, I'm still pretty new to svelte so I could be wrong.

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So it looks like this is caused indirectly by another error:

I noticed when debugging compilation.fileDependencies that there is a large array of files followed by a single [..., null] entry at the end. Looking further, using webpack serve --stats verbose I found this error:

ERROR in ./src/App.svelte
Module build failed (from ./node_modules/svelte-loader/index.js):
Error: PostCSS plugin autoprefixer requires PostCSS 8.
Migration guide for end-users:
https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users
    at Processor.normalize (/Users/duane/Relm/relm-wp5/node_modules/postcss/lib/processor.es6:117:15)
    at new Processor (/Users/duane/Relm/relm-wp5/node_modules/postcss/lib/processor.es6:38:25)
    at Object.postcss (/Users/duane/Relm/relm-wp5/node_modules/postcss/lib/postcss.es6:34:10)
    at process (/Users/duane/Relm/relm-wp5/node_modules/svelte-preprocess/dist/transformers/postcss.js:28:59)
    at transformer (/Users/duane/Relm/relm-wp5/node_modules/svelte-preprocess/dist/transformers/postcss.js:72:16)
    at Object.transform (/Users/duane/Relm/relm-wp5/node_modules/svelte-preprocess/dist/autoProcess.js:37:12)
    at runNextTicks (internal/process/task_queues.js:58:5)
    at processImmediate (internal/timers.js:434:9)
    at style (/Users/duane/Relm/relm-wp5/node_modules/svelte-preprocess/dist/autoProcess.js:167:37)
    at process_single_tag (/Users/duane/Relm/relm-wp5/node_modules/svelte/src/compiler/preprocess/index.ts:160:21)

When I add yarn add -D [email protected] and then yarn remove optimize-css-assets-webpack-plugin (and remove references to OptimizeCSSAssetsPlugin in webpack.config.js) then everything "works". Of course, this solution causes the CSS minification in prod build to be disabled, however.

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I'm able to see this too, but only with yarn. Everything is normal with npm and pnpm.

I determined from #4 that yarn is installing dependencies differently than npm which is causing inconsistencies. This is probably the same case. I'll look deeper into this when I have a chance tonight.

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Thanks @edwardrubio, your PR did fix this. The issue would have been peer dependencies defined in optimize-css-assets-webpack-plugin. I'm going to explicitly add postcss to the dev dependencies to prevent the issue from reoccurring in the future.

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