Fix ContainerRepair patch to prevent NPE (#3553)

src/test/java/net/minecraftforge/test/IsBookEnchantableTest.java

@@ -0,0 +1,61 @@
+package net.minecraftforge.test;
+
+import net.minecraft.creativetab.CreativeTabs;
+import net.minecraft.enchantment.Enchantment;
+import net.minecraft.item.Item;
+import net.minecraft.item.ItemStack;
+import net.minecraftforge.event.RegistryEvent;
+import net.minecraftforge.fml.common.Mod;
+import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
+
+@Mod(modid = IsBookEnchantableTest.MOD_ID, name = "Test for isBookEnchantable", version = "1.0")
+@Mod.EventBusSubscriber
+public class IsBookEnchantableTest
+{
+    static final String MOD_ID = "is_book_enchantable_test";
+
+    private static final Item TEST_ITEM = new TestItem();
+
+    @SubscribeEvent
+    public static void registerItem(RegistryEvent.Register<Item> event)
+    {
+        event.getRegistry().register(TEST_ITEM);
+    }
+
+    private static class TestItem extends Item
+    {
+        private static final String NAME = "test_item";
+
+        private TestItem()
+        {
+            maxStackSize = 1;
+            setUnlocalizedName(MOD_ID + "." + NAME);
+            setRegistryName(NAME);
+            setCreativeTab(CreativeTabs.MISC);
+        }
+
+        @Override
+        public boolean isEnchantable(ItemStack stack)
+        {
+            return true;
+        }
+
+        @Override
+        public int getItemEnchantability()
+        {
+            return 15;
+        }
+
+        @Override
+        public boolean isBookEnchantable(ItemStack stack, ItemStack book)
+        {
+            return false;
+        }
+
+        @Override
+        public boolean canApplyAtEnchantingTable(ItemStack stack, Enchantment enchantment)
+        {
+            return true;
+        }
+    }
+}