Check that item in hand is not null after book event. Fixes BUKKIT-5443

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -709,7 +709,7 @@ public class CraftEventFactory {
         ItemStack itemInHand = player.inventory.getItem(itemInHandIndex);
 
         // If they've got the same item in their hand, it'll need to be updated.
-        if (itemInHand.getItem() == Items.BOOK_AND_QUILL) {
+        if (itemInHand != null && itemInHand.getItem() == Items.BOOK_AND_QUILL) {
             if (!editBookEvent.isCancelled()) {
                 CraftItemStack.setItemMeta(itemInHand, editBookEvent.getNewBookMeta());
                 if (editBookEvent.isSigning()) {