Save members list when adding new members

plugin/src/main/java/com/alttd/cometskyblock/island/Island.java

@@ -125,6 +125,7 @@ public class Island extends YamlConfiguration {
         set("island.members", members.stream()
                 .map(uuid -> uuid == null ? "null" : uuid.toString())
                 .collect(Collectors.toList()));
+        save();
     }
 
     private String uuidToString(UUID uuid) {