Describe the bug
All attempts to buy an item that's for sale break, but it breaks in different ways depending on whether you purchase by ID or by creating a buy offer.

ID:

• Offer gets removed
• Buyer loses the correct amount of money
• Buyer does not get the item
• Seller does not get any money

This deletes the item and the money

Buy offer:

• Sell offer remains
• Buyer does not lose money
• Buyer gets the item
• Seller gets the money

This duplicates the item and the money

To Reproduce
Steps to reproduce the behavior:
Buy an item using a buy offer or the ID

Logs/Screenshots/Videos

ID:

[01:36:22] [Server thread/WARN] [minecraft/CommandHandler]: Couldn't process command: ge buy 0
java.lang.NullPointerException: null
at the_fireplace.grandexchange.db.JsonDatabase.removeOfferStatusComplete(JsonDatabase.java:144) ~[JsonDatabase.class:?]
at the_fireplace.grandexchange.market.OfferStatusMessager.sendCompleteStatusUpdate(OfferStatusMessager.java:66) ~[OfferStatusMessager.class:?]
at the_fireplace.grandexchange.market.OfferStatusMessager.updateStatusComplete(OfferStatusMessager.java:43) ~[OfferStatusMessager.class:?]
at the_fireplace.grandexchange.market.OfferStatusMessager.updateStatusComplete(OfferStatusMessager.java:35) ~[OfferStatusMessager.class:?]
at the_fireplace.grandexchange.commands.CommandBuy.func_184881_a(CommandBuy.java:101) ~[CommandBuy.class:?]
at the_fireplace.grandexchange.commands.CommandGe.func_184881_a(CommandGe.java:54) ~[CommandGe.class:?]
at net.minecraft.command.CommandHandler.func_175786_a(CommandHandler.java:119) [bj.class:?]
at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java:91) [bj.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147361_d(NetHandlerPlayServer.java:960) [pa.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147354_a(NetHandlerPlayServer.java:939) [pa.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:37) [la.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:9) [la.class:?]
at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) [hv$1.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_181]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_181]
at net.minecraft.util.Util.func_181617_a(SourceFile:46) [h.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:723) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_181]

Buy offer:

[01:41:07] [Server thread/WARN] [minecraft/CommandHandler]: Couldn't process command: ge buy wool 0 3 100
java.lang.NullPointerException: null
at the_fireplace.grandexchange.db.JsonDatabase.removeOfferStatusComplete(JsonDatabase.java:144) ~[JsonDatabase.class:?]
at the_fireplace.grandexchange.market.OfferStatusMessager.sendCompleteStatusUpdate(OfferStatusMessager.java:66) ~[OfferStatusMessager.class:?]
at the_fireplace.grandexchange.market.OfferStatusMessager.updateStatusComplete(OfferStatusMessager.java:43) ~[OfferStatusMessager.class:?]
at the_fireplace.grandexchange.market.ExchangeManager.tryFulfillBuyOffer(ExchangeManager.java:240) ~[ExchangeManager.class:?]
at the_fireplace.grandexchange.market.ExchangeManager.tryFulfillOffer(ExchangeManager.java:173) ~[ExchangeManager.class:?]
at the_fireplace.grandexchange.market.ExchangeManager.makeOffer(ExchangeManager.java:44) ~[ExchangeManager.class:?]
at the_fireplace.grandexchange.market.ExchangeManager.makeOffer(ExchangeManager.java:57) ~[ExchangeManager.class:?]
at the_fireplace.grandexchange.commands.CommandBuy.func_184881_a(CommandBuy.java:76) ~[CommandBuy.class:?]
at the_fireplace.grandexchange.commands.CommandGe.func_184881_a(CommandGe.java:54) ~[CommandGe.class:?]
at net.minecraft.command.CommandHandler.func_175786_a(CommandHandler.java:119) [bj.class:?]
at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java:91) [bj.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147361_d(NetHandlerPlayServer.java:960) [pa.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147354_a(NetHandlerPlayServer.java:939) [pa.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:37) [la.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:9) [la.class:?]
at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) [hv$1.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_181]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_181]
at net.minecraft.util.Util.func_181617_a(SourceFile:46) [h.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:723) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_181]

Versions (please complete the following information, do NOT say "latest"):

• Minecraft: 1.12.2
• Mod Version: 1.12.2-1.3.0
• Forge Version: 1.12.2-14.23.5.2768-universal