[12:16:51 INFO]: PoppyDoppy issued server command: /mmgui create shade
[12:16:51 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'mmgui' in plugin MythicMobsSpawnerGUI v1.9
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[patched_1.12.2.jar:git-Paper-1620]
    at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:152) ~[patched_1.12.2.jar:git-Paper-1620]
    at org.bukkit.craftbukkit.v1_12_R1.CraftServer.dispatchCommand(CraftServer.java:685) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.PlayerConnection.handleCommand(PlayerConnection.java:1492) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1297) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:45) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:5) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.PlayerConnectionUtils.lambda$ensureMainThread$0(PlayerConnectionUtils.java:14) ~[patched_1.12.2.jar:git-Paper-1620]
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_352]
    at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_352]
    at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:850) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:423) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:774) ~[patched_1.12.2.jar:git-Paper-1620]
    at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:666) ~[patched_1.12.2.jar:git-Paper-1620]
    at java.lang.Thread.run(Thread.java:750) [?:1.8.0_352]
Caused by: java.lang.IllegalArgumentException: Cannot copy null stack
    at org.apache.commons.lang.Validate.notNull(Validate.java:192) ~[patched_1.12.2.jar:git-Paper-1620]
    at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:139) ~[patched_1.12.2.jar:git-Paper-1620]
    at fr.ender_griefeur99.mythicmobsspawnergui.utils.InventoryUtils.addItem(InventoryUtils.java:80) ~[?:?]
    at fr.ender_griefeur99.mythicmobsspawnergui.MobsListNewSpawnerGUI.setItems(MobsListNewSpawnerGUI.java:66) ~[?:?]
    at fr.ender_griefeur99.mythicmobsspawnergui.MobsListNewSpawnerGUI.<init>(MobsListNewSpawnerGUI.java:44) ~[?:?]
    at fr.ender_griefeur99.mythicmobsspawnergui.MythicMobsSpawnerGUICommand.onCommand(MythicMobsSpawnerGUICommand.java:67) ~[?:?]
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[patched_1.12.2.jar:git-Paper-1620]
    ... 15 more