There’s a QuickFixCmdPost event in Vim

Which you can target in an autocmd to open the quickfix window whenever a quickfix command populates the error list (e.g. :make or :grep):

autocmd QuickFixCmdPost [^l]* cwindow

Taken from https://gist.github.com/romainl/ce55ce6fdc1659c5fbc0f4224fd6ad29