org.apache.commons.lang.UnhandledException: Plugin ServerSigns v3.1.1 generated an exception while executing task 40
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()Ljava/util/Collection;
        at de.czymm.serversigns.Metrics.postPlugin(Metrics.java:335)
        at de.czymm.serversigns.Metrics.access$4(Metrics.java:326)
        at de.czymm.serversigns.Metrics$1.run(Metrics.java:244)
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftTask.run(CraftTask.java:58)
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
        ... 3 more


出现这个错误是什么问题

版本不服吧