Bukkit

What steps will reproduce the problem?
1. Well, I put the plugin in and start the server. I set up a mailbox and so did a user. I wrote in a book, used /mail [username] and it said an internal error occurred while attempting to perform this command.
2.
3.

What is the expected output? What do you see instead?

What version of the product are you using?
0.1

Do you have an error log of what happened?
24.08 18:57:58 [Server] INFO ... 15 more
24.08 18:57:58 [Server] INFO at org.bukkit.command.PluginCommand.execute(PluginCommand.java:40)
24.08 18:57:58 [Server] INFO at com.modcrafting.bookmailbox.BookMailbox.onCommand(BookMailbox.java:136)
24.08 18:57:58 [Server] INFO at org.bukkit.Location.getBlock(Location.java:82)
24.08 18:57:58 [Server] INFO Caused by: java.lang.NullPointerException
24.08 18:57:58 [Server] INFO at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
24.08 18:57:58 [Server] INFO at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:406)
24.08 18:57:58 [Server] INFO at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:474)
24.08 18:57:58 [Server] INFO at net.minecraft.server.DedicatedServer.q(DedicatedServer.java:212)
24.08 18:57:58 [Server] INFO at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:581)
24.08 18:57:58 [Server] INFO at net.minecraft.server.DedicatedServerConnection.b(SourceFile:30)
24.08 18:57:58 [Server] INFO at net.minecraft.server.ServerConnection.b(SourceFile:35)
24.08 18:57:58 [Server] INFO at net.minecraft.server.NetServerHandler.d(NetServerHandler.java:109)
24.08 18:57:58 [Server] INFO at net.minecraft.server.NetworkManager.b(NetworkManager.java:276)
24.08 18:57:58 [Server] INFO at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:44)
24.08 18:57:58 [Server] INFO at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:807)
24.08 18:57:58 [Server] INFO at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:825)
24.08 18:57:58 [Server] INFO at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:878)
24.08 18:57:58 [Server] INFO at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:492)
24.08 18:57:58 [Server] INFO at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:168)
24.08 18:57:58 [Server] INFO at org.bukkit.command.PluginCommand.execute(PluginCommand.java:42)
24.08 18:57:58 [Server] INFO org.bukkit.command.CommandException: Unhandled exception executing command 'mail' in plugin BookMailbox v0.1
24.08 18:57:58 [Server] SEVERE null

Please provide any additional information below.
Also, what is the permission for sending mail? One of my users said that they tried and don't have permission to do so

the output says that it could not read the location of the chest/sign you tried to interact with

i cant tell you how it happend but normaly that shouldnt happen