[16:41:26 ERROR]: Error occurred while enabling CoreMediumHC v1.0 Beta (Is it up to date?)
java.lang.IllegalArgumentException: Cannot copy null stack
at org.apache.commons.lang.Validate.notNull(Validate.java:192) ~[server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:135) ~[server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.inventory.ShapedRecipe.<init>(ShapedRecipe.java:31) ~[server.jar:git-Spigot-21fe707-e1ebe52]
at pl.endrju.ytcore.utils.CraftingUtil.registerRecipe(CraftingUtil.java:161) ~[?:?]
at pl.endrju.ytcore.GuildPlugin.onEnable(GuildPlugin.java:97) ~[?:?]
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:321) ~[server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:340) [server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:405) [server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.craftbukkit.v1_8_R3.CraftServer.loadPlugin(CraftServer.java:357) [server.jar:git-Spigot-21fe707-e1ebe52]
at org.bukkit.craftbukkit.v1_8_R3.CraftServer.enablePlugins(CraftServer.java:317) [server.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.s(MinecraftServer.java:414) [server.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.k(MinecraftServer.java:378) [server.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.a(MinecraftServer.java:333) [server.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.DedicatedServer.init(DedicatedServer.java:263) [server.jar:git-Spigot-21fe707-e1ebe52]
at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:525) [server.jar:git-Spigot-21fe707-e1ebe52]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_172]