Problem/Bug Error while using shop/gui plugin

hello, i am a new server owner and i am experiencing problems when players interact with the shop. The items behave much like regular items and they can be dragged into your inventory. i will put a spoiler with the server log below

Error (open) Error (close)

Could not pass event InventoryClickEvent to ShopPlus v1.0.3
org.bukkit.event.EventException: null
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:298) ~[craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at org.bukkit.plugin.TimedRegisteredListener.callEvent(TimedRegisteredListener.java:36) ~[craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:499) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:484) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1820) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.PacketPlayInWindowClick.a(SourceFile:33) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.PacketPlayInWindowClick.a(SourceFile:10) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.PlayerConnectionUtils$1.run(SourceFile:13) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_171]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_171]
at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:696) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:371) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:651) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:555) [craftbukkit-1.12.2.jar:git-Bukkit-6103339]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_171]
Caused by: java.lang.NullPointerException
at me.bukkit.fujinuji.gui.PriceManager.priceMangerOpen(PriceManager.java:36) ~[?:?]
at me.bukkit.fujinuji.listener.Inventrory.invInteract(Inventrory.java:91) ~[?:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_171]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_171]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_171]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_171]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:296) ~[craftbukkit-1.12.2.jar:git-Bukkit-6103339]
... 16 more

Thanks!