MystCore.java:252 = invl.saveInv(player, player.getWorld());
which is a loop to save all player inventories

[11:01:57] [Server thread/ERROR]: Error occurred while disabling MystCore v1.0 (Is it up to date?)
java.lang.NullPointerException
at me.myst.MystCore.onDisable(MystCore.java:252) ~[?:?]
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:323) ~[spigot.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.plugin.java.JavaPluginLoader.disablePlugin(JavaPluginLoader.java:359) [spigot.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.plugin.SimplePluginManager.disablePlugin(SimplePluginManager.java:424) [spigot.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.plugin.SimplePluginManager.disablePlugins(SimplePluginManager.java:417) [spigot.jar:git-Spigot-870264a-0a645a2]
at org.bukkit.craftbukkit.v1_8_R2.CraftServer.disablePlugins(CraftServer.java:335) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.stop(MinecraftServer.java:458) [spigot.jar:git-Spigot-870264a-0a645a2]
at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:590) [spigot.jar:git-Spigot-870264a-0a645a2]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_31]