Issue with #walkTo(Location)

[SocketsDev]

Issue:

#walkTo(Location)

(It happens some times, but not every time)

Code (Text):
[18:45:14 WARN]: [SimplePractice] Task #80 for SimplePractice v0.1 generated an exception
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437) ~[?:1.8.0_131]
at java.util.HashMap$KeyIterator.next(HashMap.java:1461) ~[?:1.8.0_131]
at net.techcable.npclib.ai.AIEnvironment.tick(AIEnvironment.java:18) ~[?:?]
at net.techcable.npclib.nms.ai.NMSAIEnvironment.access$000(NMSAIEnvironment.java:8) ~[?:?]
at net.techcable.npclib.nms.ai.NMSAIEnvironment$1.run(NMSAIEnvironment.java:20) ~[?:?]
at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftTask.run(CraftTask.java:71) ~[spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at org.bukkit.craftbukkit.v1_7_R4.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at net.minecraft.server.v1_7_R4.MinecraftServer.v(MinecraftServer.java:641) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at net.minecraft.server.v1_7_R4.DedicatedServer.v(DedicatedServer.java:289) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at net.minecraft.server.v1_7_R4.MinecraftServer.u(MinecraftServer.java:584) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java:490) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]
at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:628) [spigot.jar:git-Spigot-1.7.9-R0.2-208-ge0f2e95]