open a host that has an interface which can not be removed (because it has items).
enter an existing group name in the "New group" field, click Update - notice how that interface can be removed now. if host has some other interfaces left, clearing "New group" field and attempting to update again fails :
Interface is linked to item "Agent ping" on "Zabbix server".
frontend should not allow to remove such interfaces after a failed update (even if api catches that properly)