Accounted for NMS getItem() returning null in recent commit.

src/main/java/net/minecraft/server/EntityTrackerEntry.java

@@ -241,6 +241,7 @@ public class EntityTrackerEntry {
 
         if (this.tracker instanceof EntityItem) {
             EntityItem entityitem = (EntityItem) this.tracker;
+            if (entityitem.dead|| entityitem.itemStack == null) return null; // CraftBukkit
             Packet21PickupSpawn packet21pickupspawn = new Packet21PickupSpawn(entityitem);
 
             entityitem.locX = (double) packet21pickupspawn.b / 32.0D;

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventory.java

@@ -27,7 +27,7 @@ public class CraftInventory implements org.bukkit.inventory.Inventory {
     }
 
     public ItemStack getItem(int index) {
-        return getInventory().getItem(index).id == 0 ? null : new CraftItemStack(getInventory().getItem(index));
+        return (getInventory().getItem(index) == null || getInventory().getItem(index).id == 0) ? null : new CraftItemStack(getInventory().getItem(index));
     }
 
     public ItemStack[] getContents() {