NorVanilla.weebly.com - TreeAssist Error - 16.05.2016

[23:11:22] [Craft Scheduler Thread - 465/WARN]: Exception in thread "Craft Scheduler Thread - 465"
[23:11:22] [Craft Scheduler Thread - 465/WARN]: org.apache.commons.lang.UnhandledException: Plugin TreeAssist v5.8.91 generated an exception while executing task 3
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()[Lorg/bukkit/entity/Player;
at me.itsatacoshop247.TreeAssist.metrics.MetricsLite.postPlugin(MetricsLite.java:284)
at me.itsatacoshop247.TreeAssist.metrics.MetricsLite.access$200(MetricsLite.java:50)
at me.itsatacoshop247.TreeAssist.metrics.MetricsLite$1.run(MetricsLite.java:172)
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftTask.run(CraftTask.java:71)
at org.bukkit.craftbukkit.v1_9_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)