Bukkit

>> What steps will reproduce the defect you are experiencing?
- go to another world? doesn't happen every time
-
-

>> What do you expect to happen, and what happens instead?
not crash, crashed

>> What version of the plugin are you using?
-latest

>> Please provide any additional information below.
Error log:
[20:08:14] [Server thread/WARN]: [DPets] Task #210939 for DPets v1.1.9 generated an exception
java.lang.IllegalArgumentException: Cannot measure distance between minenether and world
    at org.bukkit.Location.distanceSquared(Location.java:451) ~[spigot.jar:git-Spigot-eb39b47-0899683]
    at org.bukkit.Location.distance(Location.java:434) ~[spigot.jar:git-Spigot-eb39b47-0899683]
    at me.firebreath15.dpets.TargetTask.run(TargetTask.java:40) ~[?:?]
    at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftTask.run(CraftTask.java:71) ~[spigot.jar:git-Spigot-eb39b47-0899683]
    at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [spigot.jar:git-Spigot-eb39b47-0899683]
    at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:694) [spigot.jar:git-Spigot-eb39b47-0899683]
    at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot.jar:git-Spigot-eb39b47-0899683]
    at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:623) [spigot.jar:git-Spigot-eb39b47-0899683]
    at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:526) [spigot.jar:git-Spigot-eb39b47-0899683]
    at java.lang.Thread.run(Unknown Source) [?:1.7.0_05]

Try update to java 1.8, works fine here for me