The Microtype package in the version shipped with TeX Live 2018 has a bug when compiled with LuaLaTeX. See this post: https://tex.stackexchange.com/questions/476740/microtype-error-with-lualatex-attempt-to-call-field-warning-a-nil-value
The merge request implements the fix mentioned as the solution on StackExchange.
It might make sense to include the fix with our theme. But there are also good reasons not to. Maybe a comment suffices?