Hey @Nhat_Nguyen,
Apologies for the delay in the response, the reason you get undefined is because the dependencies used in the custom config are not available in the project, for eg the following are the dependencies used in the custom webpack config,
html-webpack-plugin clean-webpack-plugin mini-css-extract-plugin
you have to install these dependencies in the project root.
npm i -D html-webpack-plugin clean-webpack-plugin mini-css-extract-plugin
please let us know if this works