Could not pass event PlayerJoinEvent to GroupSystem v1.0
org.bukkit.event.EventException
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[server.jar:git-Spigot-e4d4710-e1ebe52]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[server.jar:git-Spigot-e4d4710-e1ebe52]
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PlayerList.onPlayerJoin(PlayerList.java:298) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PlayerList.a(PlayerList.java:157) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.LoginListener.b(LoginListener.java:144) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.LoginListener.c(LoginListener.java:54) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.NetworkManager.a(NetworkManager.java:231) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.ServerConnection.c(ServerConnection.java:148) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:814) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [server.jar:git-Spigot-e4d4710-e1ebe52]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_231]
Caused by: java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_231]
        at java.util.ArrayList$Itr.next(Unknown Source) ~[?:1.8.0_231]
        at ru.stonlex.groups.bukkit.database.PlayerDatabase.loadPlayer(PlayerDatabase.java:156) ~[?:?]
        at ru.stonlex.groups.bukkit.listener.DatabaseListener.onJoin(DatabaseListener.java:50) ~[?:?]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_231]
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_231]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_231]
        at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_231]
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[server.jar:git-Spigot-e4d4710-e1ebe52]
        ... 14 more