在使用jeecgboot v2.2.1版本的vue项目npm run build打包时候，前端vue打包报错。但是使用npm run serve 运行项目是可以成功的。

可能是因为 mini-css-extract-plugin 插件有问题

Building for production... ERROR  TypeError: Class extends value undefined is not a constructor or null
TypeError: Class extends value undefined is not a constructor or null
    at Object.<anonymous> (E:\************\node_modules\mini-css-extract-plugin\dist\CssDependency.js:12:46)
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:790:14)
    at Module.require (internal/modules/cjs/loader.js:974:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (E:\***********\node_modules\mini-css-extract-plugin\dist\index.js:12:45)

解决方案：
npm add webpack@4.4.1

PS：package-lock.json和node_modules文件夹全部删除，重新install