Laravel 6 NPM run dev 之后, app.css 是空的

問題：

sidebarMenu.js 中 的 menu comment 拿掉时，
run dev 之后, app.css 就变成空档案

sidebarMenu.js↓

const app = new Vue({
    el: '#sidebarMenu',
    components: {
        sideBar: () => require('./components/sidebarMenu/main').default,
        // Menu: () = required('./components/sidebarMenu/menu')
    }
});

webpack.mix.js↓

mix
    .js('resources/js/pages/dashboard.js', 'public/js')
    .js('resources/js/sidebarMenu.js', 'public/js')
    .js('resources/js/app.js', 'public/js')
    .js('resources/js/bootstrap.js', 'public/js')

mix.sass('resources/sass/app.scss', 'public/css').version();

package.json↓

{
    "private": true,
    "scripts": {
        "dev": "npm run development",
        '略...'
    },
    "devDependencies": {
        "autoprefixer-loader": "^3.2.0",
        "axios": "^0.19",
        "babel-preset-vue": "^2.0.2",
        "cross-env": "^7.0",
        "css-loader": "^3.5.3",
        "dotenv-webpack": "^1.8.0",
        "jquery": "^3.5.1",
        "laravel-mix": "^5.0.1",
        "lodash": "^4.17.13",
        "resolve-url-loader": "^3.1.0",
        "sass": "^1.26.7",
        "sass-loader": "^8.0.0",
        "style-loader": "^1.2.1",
        "vue-template-compiler": "^2.6.11"
    },
    "dependencies": {
        "@babel/core": "^7.10.2",
        "@babel/preset-env": "^7.10.2",
        "@fortawesome/fontawesome-free": "^5.13.0",
        "admin-lte": "^3.0.5",
        "autoprefixer": "^9.8.0",
        "babel-loader": "^8.1.0",
        "popper": "^1.0.1",
        "v-select": "0.0.1",
        "vue": "^2.6.11",
        "vue-sweetalert2": "^3.0.5"
    }
}

menu 的 comment 拿掉之后的异常↓

正常状况↓

想请问是缺少设定什么地方吗?