Hello, you can drop an item out of a GUI and it'll be placed into your inventory.

PlayerListener line 179 evt.getWhoClicked().closeInventory(); according to https://hub.spigotmc.org/javadocs/spigot/org/bukkit/event/inventory/InventoryClickEvent.html
We should not be using the entity that invoked the method, could this be the problem.

Please update to the latest build of Quests, then copy and paste the error in its entirety.

I will try this again soon, but we added a few checks to disable drops and it worked. I could make a PR but I don't think it's a 'standard' way to fix the issue, it was more of a hot fix.

I'll try the latest one shortly though.

@AshourAziz I believe this could have been fixed in #343. If this is still an issue after updating to Quests 2.6.4, paste the new error in a new issue along with your quests.yml file. Thanks!