Wrap firing ServerCustomPayloadEvent in ensureInEventLoop

core/src/main/java/org/geysermc/geyser/translator/protocol/java/JavaCustomPayloadTranslator.java

@@ -141,8 +141,10 @@ public class JavaCustomPayloadTranslator extends PacketTranslator<ClientboundCus
                 session.sendUpstreamPacket(toSend);
             });
         } else {
+            session.ensureInEventLoop(() -> {
                 EventBus eventBus = session.getGeyser().eventBus();
                 eventBus.fire(new ServerCustomPayloadEvent(session, channel, packet.getData()));
+            });
         }
     }