Exception in thread "Timer-3"
[07:59:26 WARN]: java.lang.IllegalStateException: Asynchronous entity add!

[07:59:26 WARN]:        at org.spigotmc.AsyncCatcher.catchOp(AsyncCatcher.java:14)

[07:59:26 WARN]:        at net.minecraft.server.v1_11_R1.World.addEntity(World.java:1013)

[07:59:26 WARN]:        at net.minecraft.server.v1_11_R1.WorldServer.addEntity(WorldServer.java:1065)

[07:59:26 WARN]:        at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.addEntity(CraftWorld.java:1203)

[07:59:26 WARN]:        at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.spawn(CraftWorld.java:1210)

[07:59:26 WARN]:        at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.spawn(CraftWorld.java:854)

[07:59:26 WARN]:        at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.spawnEntity(CraftWorld.java:356)

[07:59:26 WARN]:        at de.league.mobs.Skelett$1.run(Skelett.java:24)

[07:59:26 WARN]:        at java.base/java.util.TimerThread.mainLoop(Timer.java:556)

[07:59:26 WARN]:        at java.base/java.util.TimerThread.run(Timer.java:506)