left bug report on citizens2 github and fullwall said its a hypermerchant issue. here is the details.
[09:02:42] [Server thread/ERROR]: Could not pass event NPCRightClickEvent to Citizens v2.0.17-SNAPSHOT (build 1319)
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.citizensnpcs.EventListen.onPlayerInteractEntity(EventListen.java:386) [citizens-2.0.17-SNAPSHOT.jar:?]
at sun.reflect.GeneratedMethodAccessor311.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_73]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_73]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.PlayerConnection.a(PlayerConnection.java:1524) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.PacketPlayInUseEntity.a(SourceFile:68) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.PacketPlayInUseEntity.a(SourceFile:12) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_73]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_73]
at net.minecraft.server.v1_9_R1.SystemUtils.a(SourceFile:45) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:721) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:400) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:660) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:559) [spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_73]
Caused by: java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Unknown Source) ~[?:1.8.0_73]
at java.lang.Short.parseShort(Unknown Source) ~[?:1.8.0_73]
at java.lang.Short.parseShort(Unknown Source) ~[?:1.8.0_73]
at regalowl.hyperconomy.inventory.HItemStack.(HItemStack.java:49) ~[?:?]
at regalowl.hyperconomy.tradeobject.BasicTradeObject.getItem(BasicTradeObject.java:705) ~[?:?]
at regalowl.hyperconomy.tradeobject.BasicTradeObject.getItemStack(BasicTradeObject.java:709) ~[?:?]
at grokswell.hypermerchant.ShopMenu.loadPage(ShopMenu.java:192) ~[?:?]
at grokswell.hypermerchant.ShopMenu.(ShopMenu.java:115) ~[?:?]
at grokswell.hypermerchant.HyperMerchantTrait.onRightClick(HyperMerchantTrait.java:367) ~[?:?]
at sun.reflect.GeneratedMethodAccessor529.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_73]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_73]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot-1.9.jar:git-Spigot-2dc26f3-4a8818c]
... 23 more

it happens seemingly for no reason when right clicking on a shop. it stops the gui from opening. when i disable the mod the gui opens blank. full wall said it might be hypermerchant trying to open or load something.

Looks like you are running Spigot 1.9
I haven't released a build of HyperMerchant yet for that version. I will try to this weekend

This issue is fixed since HyperMerchant 1.6-dev