This is the error I get when I try to check player's permission. And this is the method I'm using.

There is only one player at the server and that player is op. (via LuckPerms' auto op option)

Forgot to mention, I'm using latest development build version. (v3.2.42)

permission is null.

It is Bukkit behaviour to throw an exception if the permission is null, so it's not an LP issue.
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/browse/src/main/java/org/bukkit/permissions/PermissibleBase.java#66

Ahh silly me... Thanks for pointing out the obvious.