NPE - Cannot invoke "org.bukkit.entity.Player.getUniqueId()" because "player" is null

Stacktrace
```
java.lang.NullPointerException: Cannot invoke "org.bukkit.entity.Player.getUniqueId()" because "player" is null
        at com.projectkorra.items.PKItem.updateStack(PKItem.java:172) ~[ProjectKorraItems-2.0-PK1.9.3.jar:?]
        at com.strangeone101.holoitemsapi.listener.ItemListener.onItemSpawn(ItemListener.java:326) ~[ProjectKorraItems-2.0-PK1.9.3.jar:?]
```