When using "Add field" for custom item types keep the list open until all selections are made, then click to close.

When you're trying to add custom fields to a custom item type keep the selectable options open until all selections have been made then click off of the list to close the list of selectable items. This will prevent the need for the user to manually select "Add field" repetitively until all desired custom fields have been added and streamline the creation or updating of custom item types.