Bukkit

When a player interacts with a sign im getting this error. Also, people without OP can't interact with the signs. Why?

Could not pass event PlayerInteractEvent to SignWarper v2.0

org.bukkit.event.EventException

at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[server.jar:git-Spigot-db6de12-18fbb24]

at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[server.jar:git-Spigot-db6de12-18fbb24]

at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [server.jar:git-Spigot-db6de12-18fbb24]

at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [server.jar:git-Spigot-db6de12-18fbb24]

at org.bukkit.craftbukkit.v1_8_R3.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:228) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.PlayerInteractManager.interact(PlayerInteractManager.java:463) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:759) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:52) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:1) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [server.jar:git-Spigot-db6de12-18fbb24]

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_181]

at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_181]

at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [server.jar:git-Spigot-db6de12-18fbb24]

at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [server.jar:git-Spigot-db6de12-18fbb24]

at java.lang.Thread.run(Thread.java:748) [?:1.8.0_181]

Caused by: java.lang.NoSuchFieldError: ENTITY_ENDERDRAGON_FLAP

at me.powermais.signwarper.Utils.playTeleportSound(Utils.java:77) ~[?:?]

at me.powermais.signwarper.Events.onPlayerInteract(Events.java:69) ~[?:?]

at sun.reflect.GeneratedMethodAccessor32.invoke(Unknown Source) ~[?:?]

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_181]

at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_181]

at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[server.jar:git-Spigot-db6de12-18fbb24]

... 17 more