Make sure theres a stack to split before attempting.

src/main/java/com/songoda/ultimatestacker/stackable/entity/ColdEntityStack.java

@@ -87,6 +87,7 @@ public class ColdEntityStack implements Stackable {
     }
 
     public LivingEntity takeOneAndSpawnEntity(Location location) {
+        if (stackedEntities.isEmpty()) return null;
         NBTEntity nbtEntity = NmsManager.getNbt().newEntity();
         nbtEntity.deSerialize(stackedEntities.getFirst().getSerializedEntity());
         LivingEntity newEntity = (LivingEntity)nbtEntity.spawn(location);

src/main/java/com/songoda/ultimatestacker/tasks/StackingTask.java

@@ -321,6 +321,7 @@ public class StackingTask extends BukkitRunnable {
         Bukkit.getScheduler().runTask(plugin, () -> {
             for (int i = stackSize; i > 0; i -= maxEntityStackAmount) {
                 LivingEntity entity = stack.takeOneAndSpawnEntity(livingEntity.getLocation());
+                if (entity == null) continue;
                 EntityStack newStack = plugin.getEntityStackManager().addStack(entity);
                 newStack.moveEntitiesFromStack(stack, Math.min(i, maxEntityStackAmount) - 1);
                 newStack.updateStack();