Android - "Fastboot devices" command doesn't work after macos high sierra 10.14.4 upgrade

This might be a regression in android-platform-tools. Try fastboot version 9dc0875966c0-android from android-platform-tools 26.0.1 as it works for me on macOS 10.14.4

I had the same error in version 28.0.3 but it's fixed in 29.0.1. You can download it from the official site.
It's also available from MacPorts, e.g

port install android-platform-tools

This is a Fastboot problem which has been fixed by google. See the release notes here:
SDK Platform Tools release notes

Here is what solved my problem:
Start android studio->go to sdk manager->select SDK Platform Tools -> Check "Show Details" -> Select Android Platform Tools -> Upgrade to 29.0.1 or newer.

See the images for more details: