Changing `customEmoji` incorrectly decrements current active nav button

[nolanlawson]

1. Got to https://nolanlawson.github.io/emoji-picker-element/
2. Click on the 🍎 to change to the food category
3. Click the "custom emoji" checkbox
4. Now the "animals" category will be selected instead

Probably doesn't matter much since most emoji pickers will have the custom category either set or unset when the component first loads.