What is an access widener?

I keep getting this error.

Uncaught exception in thread "main"

java.lang.RuntimeException: Missing accessWidener file emc.accesswidener from mod emc

at net.fabricmc.loader.impl.FabricLoaderImpl.loadAccessWideners(FabricLoaderImpl.java:509)

at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:149)

at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)

at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

It says that an access widener file is missing. What do I do?