Whenever you hash certain books, this exception happens.

I think there is just a null check missing somewhere, the expected behavior is that you are able to hash this any valid item.

[13:33:24] [Server thread/ERROR]: Could not pass event PlayerInteractEvent to CustomPlugin
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:305) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.craftbukkit.v1_8_R1.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:226) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.PlayerInteractManager.interact(PlayerInteractManager.java:463) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:724) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:50) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:80) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [?:1.7.0_45]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [?:1.7.0_45]
at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:683) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:623) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:526) [spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at java.lang.Thread.run(Thread.java:744) [?:1.7.0_45]
Caused by: java.lang.NullPointerException
at net.minecraft.server.v1_8_R1.ChatBaseComponent.hashCode(SourceFile:117) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at java.util.AbstractList.hashCode(AbstractList.java:541) ~[?:1.7.0_45]
at org.bukkit.craftbukkit.v1_8_R1.inventory.CraftMetaBook.applyHash(CraftMetaBook.java:287) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.craftbukkit.v1_8_R1.inventory.CraftMetaBookSigned.applyHash(CraftMetaBookSigned.java:117) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.craftbukkit.v1_8_R1.inventory.CraftMetaItem.hashCode(CraftMetaItem.java:736) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at org.bukkit.inventory.ItemStack.hashCode(ItemStack.java:368) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
at com.pluginpackage.PlayerListener.checkHash(PlayerListener.java:151) ~[?:?]
at com.pluginpackage.PlayerListener.use(PlayerListener.java:217) ~[?:?]
at sun.reflect.GeneratedMethodAccessor455.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.7.0_45]
at java.lang.reflect.Method.invoke(Method.java:606) ~[?:1.7.0_45]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[spigot-1.8%20(3).jar:git-Spigot-13716d9-0899683]
... 16 more