This line might be the reason the client is kicked from the server when the event fires.
As far as I can tell without being a mod dev this is the client instance:
Minecraft.getInstance()

https://github.com/DroidDude/Flight-Potion/blob/9371a6906b9ecf0e50386af69004d2fba6b5ba20/src/main/java/eu/newaustrianservers/fltpot/event/Events.java#L40

Well that's a dupe. See #2