I have released Protobuf v3.12.0-rc2 to GitHub and the per-language repositories (NuGet, npm, RubyGems, etc).
https://github.com/protocolbuffers/protobuf/releases/tag/v3.12.0-rc2
This fixes a few issues that were reported in 3.12.0-rc1 (see the changelog above for more details).
Unless there are significant issues reported in the next few days, I'll follow with a 3.12.0 later this week.