第二种打包import的css报错

      {

        test: /\.css$/,

        //user可为loader加入选项

        use: [

          'style-loader',

          {loader: 'css-loader',options: {importLoaders: 1}},

          {loader: 'postcss-loader',options: {plugins: [require('autoprefixer')({browsers:["last 5 versions"]})]}}

        ],

      }

ERROR in ./node_modules/css-loader?{"importLoaders":1}!./node_modules/postcss-loader/lib?{"plugins":[null]}!./src/css/flex.css

Module build failed: TypeError: Cannot read property 'postcss' of null

    at Processor.normalize (F:\webpack-demo\node_modules\postcss-loader\node_modules\postcss\lib\processor.js:136:12)

    at new Processor (F:\webpack-demo\node_modules\postcss-loader\node_modules\postcss\lib\processor.js:51:25)

    at postcss (F:\webpack-demo\node_modules\postcss-loader\node_modules\postcss\lib\postcss.js:73:10)

    at Promise.resolve.then.then (F:\webpack-demo\node_modules\postcss-loader\lib\index.js:137:12)

    at <anonymous>

 @ ./node_modules/css-loader?{"importLoaders":1}!./node_modules/postcss-loader/lib?{"plugins":[null]}!./src/css/common.css 3:10-162

 @ ./src/css/common.css

 @ ./src/app.js