1.14.4 How to check if Citizens NPC exists?

How can I check if an NPC exists in a world?

I'm using citizens API.

 

I tried using this code but it can't tell if any NPCs exist.

Code (Text):

for(Entity entity: Bukkit.getWorld("world").getEntities()) {
if(CitizensAPI.getNPCRegistry().isNPC(entity)) {
System.out.println("npc exist!");
} else {
System.out.println("no npcs exist!");
}
}