title: Entrypoint undefined = extract-text-webpack-plugin-output-filename in webpack 4?
body: <p>I extracted css using extract-text-webpack-plugin in webpack 4. The CSS is exctacted but when i run npm run build , the css is extracted but i get a message on screen saying </p>
  
  <p><strong>Entrypoint undefined = extract-text-webpack-plugin-output-filename.</strong></p>
  
  <p>The Entire Stack Trace is below:</p>
  
  <pre><code>$ npm run build
  
  &gt; comp@1.0.0 build /media/azeem/A036C96E36C945CE/DevlopersConsole/Projects/comp
  &gt; webpack
  
  Hash: 81ca52e4a0297e3e830f
  Version: webpack 4.7.0
  Time: 52303ms
  Built at: 2018-05-06 23:13:09
         Asset      Size  Chunks             Chunk Names
     bundle.js  4.06 KiB    main  [emitted]  main
  css/main.css  58 bytes    main  [emitted]  main
  Entrypoint main = bundle.js css/main.css
  [./node_modules/css-loader/index.js!./node_modules/postcss-loader/lib/index.js!./node_modules/sass-loader/lib/loader.js!./src/main.scss] ./node_modules/css-loader!./node_modules/postcss-loader/lib!./node_modules/sass-loader/lib/loader.js!./src/main.scss 197 bytes [built]
  [./node_modules/css-loader/index.js!./node_modules/postcss-loader/lib/index.js!./node_modules/sass-loader/lib/loader.js!./src/one.scss] ./node_modules/css-loader!./node_modules/postcss-loader/lib!./node_modules/sass-loader/lib/loader.js!./src/one.scss 183 bytes [built]
  [./src/app.js] 162 bytes {main} [built]
  [./src/main.scss] 41 bytes [built]
  [./src/one.scss] 41 bytes [built]
  [0] multi ./src/app.js ./src/main.scss 40 bytes {main} [built]
      + 3 hidden modules
  Child extract-text-webpack-plugin node_modules/extract-text-webpack-plugin/dist node_modules/css-loader/index.js!node_modules/postcss-loader/lib/index.js!node_modules/sass-loader/lib/loader.js!src/main.scss:
      Entrypoint undefined = extract-text-webpack-plugin-output-filename
      [./node_modules/css-loader/index.js!./node_modules/postcss-loader/lib/index.js!./node_modules/sass-loader/lib/loader.js!./src/main.scss] ./node_modules/css-loader!./node_modules/postcss-loader/lib!./node_modules/sass-loader/lib/loader.js!./src/main.scss 197 bytes {0} [built]
          + 1 hidden module
  Child extract-text-webpack-plugin node_modules/extract-text-webpack-plugin/dist node_modules/css-loader/index.js!node_modules/postcss-loader/lib/index.js!node_modules/sass-loader/lib/loader.js!src/one.scss:
      Entrypoint undefined = extract-text-webpack-plugin-output-filename
      [./node_modules/css-loader/index.js!./node_modules/postcss-loader/lib/index.js!./node_modules/sass-loader/lib/loader.js!./src/one.scss] ./node_modules/css-loader!./node_modules/postcss-loader/lib!./node_modules/sass-loader/lib/loader.js!./src/one.scss 183 bytes {0} [built]
  </code></pre>
  
  <p><strong>Package.json</strong> file is:</p>
  
  <pre><code>{
    "name": "comp",
    "version": "1.0.0",
    "description": "",
    "main": "webpack.config.js",
    "dependencies": {},
    "devDependencies": {
      "babel-core": "^6.26.3",
      "babel-loader": "^7.1.4",
      "babel-preset-env": "^1.6.1",
      "css-loader": "^0.28.11",
      "extract-text-webpack-plugin": "^4.0.0-beta.0",
      "mini-css-extract-plugin": "^0.4.0",
      "node-sass": "^4.9.0",
      "postcss-loader": "^2.1.5",
      "precss": "^3.1.2",
      "sass-loader": "^7.0.1",
      "style-loader": "^0.21.0",
      "uglifyjs-webpack-plugin": "^1.2.5",
      "webpack": "^4.7.0",
      "webpack-cli": "^2.1.2"
    },
    "scripts": {
      "test": "echo \"Error: no test specified\" &amp;&amp; exit 1",
      "build": "webpack",
      "start": "webpack-dev-server"
    },
    "keywords": [],
    "author": "",
    "license": "ISC"
  }
  </code></pre>
  
  <p><strong>webpack.config.js</strong></p>
  
  <pre><code>// const webpack = require('webpack');
  const path = require('path');
  
  const ExtractTextPlugin = require("extract-text-webpack-plugin");
  const extractCSS = new ExtractTextPlugin({
      filename: 'css/main.css',
  
  });
  
  /*
   * We've enabled UglifyJSPlugin for you! This minifies your app
   * in order to load faster and run less javascript.
   *
   * https://github.com/webpack-contrib/uglifyjs-webpack-plugin
   *
   */
  
  const MiniCssExtractPlugin = require("mini-css-extract-plugin");
  
  
  
   const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
  
  /*
   * SplitChunksPlugin is enabled by default and replaced
   * deprecated CommonsChunkPlugin. It automatically identifies modules which
   * should be splitted of chunk by heuristics using module duplication count and
   * module category (i. e. node_modules). And splits the chunks…
   *
   * It is safe to remove "splitChunks" from the generated configuration
   * and was added as an educational example.
   *
   * https://webpack.js.org/plugins/split-chunks-plugin/
   *
   */
  
  module.exports = {
      entry : [ './src/app.js', './src/main.scss'],
      output: {
          filename: 'bundle.js',
  
          path: path.resolve(__dirname, 'dist'),
          publicPath: '/dist',
      },
  
      module: {
          rules: [
              {
                  test: /\.js$/,
                  exclude: /node_modules/,
                  loader: 'babel-loader',
  
                  options: {
                      presets: ['env']
                  }
              },
              {
                  test: /\.(scss|css)$/,
  
                  use: extractCSS.extract({
                      fallback: 'style-loader',
                      use: [
                       { loader: 'css-loader'},
                       { loader: 'postcss-loader' },
                       { loader: 'sass-loader' }
                      ],
                      filename : 'mains.css',
                      publicPath: '/build/'
  
                  })
              }
          ]
      },
  
      plugins: [
  
      extractCSS,
  
      ],
      mode: 'development',
  
      optimization: {
          splitChunks: {
              chunks: 'async',
              minSize: 30000,
              minChunks: 1,
              name: true,
  
              cacheGroups: {
                  vendors: {
                      test: /[\\/]node_modules[\\/]/,
                      priority: -10
                  }
              }
          }
      },
  
      devServer:{
          contentBase: './'
      }
  
  };
  </code></pre>