Quests Illegal Argument Exception

[13:23:41] [Server thread/WARN]: [Quests] Task #100 for Quests v3.4.2-b500 generated an exception
java.lang.IllegalArgumentException: Material cannot be null
    at org.apache.commons.lang.Validate.notNull(Validate.java:192) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:69) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:57) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:45) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:35) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at me.blackvein.quests.util.ItemUtil.readItemStack(ItemUtil.java:112) ~[?:?]
    at me.blackvein.quests.Quests.loadQuestRequirements(Quests.java:2338) ~[?:?]
    at me.blackvein.quests.Quests.loadQuests(Quests.java:2176) ~[?:?]
    at me.blackvein.quests.Quests$1.run(Quests.java:296) ~[?:?]
    at org.bukkit.craftbukkit.v1_13_R2.scheduler.CraftTask.run(CraftTask.java:76) ~[spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at org.bukkit.craftbukkit.v1_13_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:361) [spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at net.minecraft.server.v1_13_R2.MinecraftServer.b(MinecraftServer.java:893) [spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at net.minecraft.server.v1_13_R2.DedicatedServer.b(DedicatedServer.java:417) [spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at net.minecraft.server.v1_13_R2.MinecraftServer.a(MinecraftServer.java:835) [spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at net.minecraft.server.v1_13_R2.MinecraftServer.run(MinecraftServer.java:733) [spigot_latest.jar:git-Spigot-5696c83-c2035aa]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0_162]