Don't try and stack non stackable items.

src/main/java/com/songoda/ultimatestacker/listeners/ItemListeners.java

@@ -95,6 +95,7 @@ public class ItemListeners implements Listener {
 
         Item item = event.getItem();
         ItemStack stack = item.getItemStack();
+        if (stack.getMaxStackSize() == 1) return;
         int amount = UltimateStacker.getActualItemAmount(item);
         if (amount < (stack.getMaxStackSize() / 2)) return;
 

src/main/java/com/songoda/ultimatestacker/utils/EntityUtils.java

@@ -13,7 +13,6 @@ import org.bukkit.entity.*;
 import org.bukkit.inventory.EntityEquipment;
 
 import java.util.*;
-import java.util.stream.Collectors;
 
 public class EntityUtils {