The full error is:
> [webpack-cli] Invalid options object. Dev Server has been initialized using an options object that does not match the API schema.
- options has an unknown property '_assetEmittingPreviousFiles'. These properties are valid:
object { allowedHosts?, bonjour?, client?, compress?, devMiddleware?, headers?, historyApiFallback?, host?, hot?, ipc?, liveReload?, onListening?, open?, port?, proxy?, server?, setupExitSignals?, setupM
iddlewares?, static?, watchFiles?, webSocketServer? }