Skywars Plugin Error

Currently I am trying to start a server on Minecraft version 1.8.8 centered around the classic Skywars plugin.
However, whenever I attempt to start a game of Skywars, it always returns this error:

Code (Text):

[18:57:54 INFO]: Benokt issued server command: /sw start
[18:57:54 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'sw' in plugin SkyWars v1.1-m16
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) ~[spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_66]
        at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_66]
        at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot.jar:git-Spigot-e4d4710-e1ebe52]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_66]
Caused by: java.lang.NullPointerException
        at vc.pvp.skywars.commands.StartCommand.onCommand(StartCommand.java:21) ~[?:?]
        at vc.pvp.skywars.commands.MainCommand.onCommand(MainCommand.java:53) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-e4d4710-e1ebe52]
        ... 15 more

Is anyone capable of deciphering this? To me it resembles some sort of compatibility issue.

 

Correct me if I'm wrong, but shouldn't all versions of 1.8 be compatible with each other?