MQTT.fx crashed with NullpointerException on start-up on MacOS 10.13 High Sierra.
Unfortunately this version have to show the MenuBar within the Application window (like a Windows™ App):
Not happy with that at all, will find a way to fix this in MQTT.fx 1.6.0!
As always you can download latest binaries/installers at mqttfx.org.
Please Report bugs and feature requests via the issue tracker.