How do i compile this specific branch bugfix from GitHub

Specifically the new bug fix for 1.12.2 Simple Voice Chat.
I showed the bug to the dev Henkelmax and he was able to fix it, its just that hes too busy to release it publically and told me to "compile it myself"

i tried my best with Github by downloading the zip for 1.12.2 branch, then extracting and opening cmd in the folder, jdk 8, then gradlew build fails saying javadoc cant be generated. and im stumped.

This was the specific bugfix: https://github.com/henkelmax/simple-voice-chat/issues/897

would appreciate all help, cuz i need this server side mod asap.