When opening an inventory (for example player inventory), taking any item into the hand and clicking in an non-slot part of the ui (not outside and not on a slot) will throw an exception when trying to getCurrentItem() in the InventoryClickEvent:

@EventHandler
public void onClick(InventoryClickEvent event) {
    event.getCurrentItem(); //throws exception
}

throws

java.lang.ArrayIndexOutOfBoundsException: -1
        at java.util.ArrayList.elementData(Unknown Source) ~[?:1.8.0_172]
        at java.util.ArrayList.get(Unknown Source) ~[?:1.8.0_172]
        at net.minecraft.server.v1_13_R1.Container.getSlot(Container.java:110) ~[spigot-1.13-pre7.jar:git-Spigot-1d0aef7-b4bff6f]
        at org.bukkit.craftbukkit.v1_13_R1.inventory.CraftInventoryView.getItem(CraftInventoryView.java:65) ~[spigot-1.13-pre7.jar:git-Spigot-1d0aef7-b4bff6f]
        at org.bukkit.event.inventory.InventoryClickEvent.getCurrentItem(InventoryClickEvent.java:113) ~[spigot-1.13-pre7.jar:git-Spigot-1d0aef7-b4bff6f]

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