getPermPrefix should return the actual prefix and not null

src/main/java/bentobox/addon/warps/Warp.java

@@ -97,8 +97,7 @@ public class Warp extends Addon {
     }
 
     public String getPermPrefix(World world) {
-        this.getPlugin().getIWM().getPermissionPrefix(world);
-        return null;
+        return this.getPlugin().getIWM().getPermissionPrefix(world);
     }
 
     /**