Interesting parts of the NPE when closing the invisible inventory:
at org.bukkit.craftbukkit.v1_8_R2.event.CraftEventFactory.handleInventoryCloseEvent(CraftEventFactory.java:838) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PlayerConnection.a(PlayerConnection.java:1401) [spigot.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.craftbukkit.v1_8_R2.event.CraftEventFactory.callInventoryOpenEvent(CraftEventFactory.java:706) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.EntityPlayer.openContainer(EntityPlayer.java:663) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.BlockFurnace.interact(SourceFile:114) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PlayerInteractManager.interact(PlayerInteractManager.java:496) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PlayerConnection.a(PlayerConnection.java:740) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:52) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:1) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-870264a-0a645a2]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_05]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_05]
at net.minecraft.server.v1_8_R2.SystemUtils.a(SourceFile:60) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:712) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [spigot.jar:git-Spigot-870264a-0a645a2]

Done a small amount of digging: after calling the openInventory method for some Player, when a InventoryCloseEvent is thrown when that invisible inventory is closed its holder appears to be null.

Also having this issue with CraftBukkit version git-spigot-870264a-0a645a2 (MC: 1.8.3) (Implementing API version 1.8.3-R0.1-SNAPSHOT). Nothing comes up for the player when the .openInventory method is called, and when they hit the ESC key I get errors regarding the InventoryClose event (can provide some code if you want, just saying as it suggests a furnace is partly opened).

Spigot dont want to fix it....

its related to openInventory(). It opens the Inventory but the client doesn´t show it, when you use 1.8 Client, and 1.7 Server it works fine so its a 1.8 Server bug.

To check if you can't interact with the inventory at all, or if the problem is related to openInventory().

I can't open the Inventory so why should I close it?

Does closeInventory() work?

I have the same problem! FIX IT!!

Your build is not the latest and therefore may be the reason you are having this issue. Spigot is 0 version(s) behind. CraftBukkit is 1 version(s) behind. This message was automatically generated and is not guaranteed to be a solution to your issue.