My Console Problem

[14:58:41 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'nick' in plugin NickNamer v0.9
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.dispatchCommand(CraftServer.java:647) ~[craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.PlayerConnection.handleCommand(PlayerConnection.java:1386) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1221) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:45) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:1) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.PlayerConnectionUtils$1.run(SourceFile:13) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_144]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_144]
	at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:748) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()[Lorg/bukkit/entity/Player;
	at bukkit.killjoy64.NickNamer.util.Messenger.notify(Messenger.java:48) ~[?:?]
	at bukkit.killjoy64.NickNamer.commands.NickExecutor.onCommand(NickExecutor.java:68) ~[?:?]
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[craftbukkit.jar:git-Spigot-65e8124-bde2a93]
	... 15 more