[12:17:36] [Server thread/INFO]: This server is running CraftBukkit version git-Spigot-1517 (MC: 1.7.9) (Implementing API version 1.7.9-R0.3-SNAPSHOT)

error log for InventoryClickEvent to EchoPet v2.3.4-SNAPSHOT-b442
http://pastebin.com/sBhjE9Uv

error log for InventoryClickEvent to EchoPet v2.4.0-SNAPSHOT-b480
http://pastebin.com/3DSm5dQ5

Yes, I get this too, It the whole plugin works fine apart from this, But it isn't too critical as it doesn't stop anything from working

As you haven't actually listed how you get this error, Due to me getting the same I though I will add it

You get this error when the GUI is open to select a pet and you click somewhere not in the Menu