Downloaded src (our current mods are for 1.7.10) forge-1.7.10-10.13.4.1614-1.7.10-src. followed the instructions on https://mcforge.readthedocs.io/en/latest/gettingstarted/, copied files to projects folder, opened an elevated command prompt, browsed to the folder and ran gradlew setupdecompworkspace. The error was A problem occurred evaluating root project 'projects'
Failed to apply plugin [ID 'forge']
You muse set the Minecraft Version!
Java.lang.nullPointerException (no error message)