public class Main extends JavaPlugin {

   @Override
   public void onEnable() {
      ItemStack head1 = new ItemStack(Material.PLAYER_HEAD);

      UUID uuid = UUID.randomUUID();
      GameProfile profile1 = new GameProfile(uuid, null);
      profile1.getProperties().put("textures", new Property("textures", "some random skin value"));

      setProfile(head1, profile1);

      ItemStack head2 = head1.clone();

      if(head1.isSimilar(head2)) { // NPE here  
          System.out.println("Is similar");  
      }

   }

   private void setProfile(ItemStack item, GameProfile profile) {
      ItemMeta meta = item.getItemMeta();
      try {
         Field field = meta.getClass().getDeclaredField("profile");
         field.setAccessible(true);
         field.set(meta, profile);
      } catch (NoSuchFieldException | IllegalAccessException ex) {
         ex.printStackTrace();
      }
      item.setItemMeta(meta);
   }
}