Bukkit

This message will be auto displayed after a while. i did't know what's happening.. 

" [CommandWatcher] Plugin CommandWatcher v0.5 generated an exception while executing task 28[CommandWatcher] Plugin CommandWatcher v0.5 generated an exception while executing task 28java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()[Lorg/bukkit/entity/Player; at me.mcluke300.commandlistener.MetricsLite.postPlugin(MetricsLite.java:234) ~[?:?] at me.mcluke300.commandlistener.MetricsLite.access$4(MetricsLite.java:225) ~[?:?] at me.mcluke300.commandlistener.MetricsLite$1.run(MetricsLite.java:143) ~[?:?] at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftTask.run(CraftTask.java:71) ~[server.jar:git-Spigot-596221b-9a1fc1e] at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:52) [server.jar:git-Spigot-596221b-9a1fc1e] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_181] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_181] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_181] "