The easiest way to install NextPVR is using the .deb file (thanks Martin for creating it!).
curl https://nextpvr.com/nextpvr-helper.deb -O sudo apt install ./nextpvr-helper.deb --install-recommends
Give it a half a minute or so to start, then you should be able to open http://127.0.0.1:8866 in your browser to setup NextPVR. The default login is admin/password.