A crash is caused when trying to offhand an item in the creative inventory after attempting to do so in the search tab

Steps to reproduce:

1. Have an item in your off-hand in creative mode
2. Open inventory and go to the Search tab
3. Use the off-hand key to try and put the item in your off-hand into the hotbar (stopped as the search box takes priority)
4. Change back to the Survival Inventory tab, and then press the off-hand key again without hovering cursor over any inventory slots

or:

1. Have an item in your off-hand in creative mode
2. Hover over any empty slot in a tab other than Survival Inventory
3. Select the Survival Inventory tab, and then press the off-hand key without hovering cursor over any inventory slots

As far as I can tell (Not a programmer or anything, didn't look at code, just from my small testing) this happens because the offhand hotkey when pressed tries to put the item in your offhand into the last inventory slot that you had "selected" (hovered over) with your cursor. When you try to use the offhand hotkey when under the search tab, the search box takes priority and types instead of allowing you to offhand, making it the last selected inventory "slot" (I believe,) when you change back to your player inventory tab, there's no textbox anymore, so when you try to use the hotkey again, the game crashes as there's no "last selected" slot to offhand to (or maybe because it tries to offhand to the textbox? I dunno I'm not a programmer)

Simply hovering over an inventory slot after swapping to the player inventory before using the offhand hotkey will prevent the crash and allow the hotkey to work as normal.