After June 30, 1972, no new drainage districts shall be organized under the provisions of sections 31-401 to 31-450. Attempted formations of drainage districts under sections 31-401 to 31-450 which have not been completed before July 1, 1972, shall be null, void and of no effect for the purpose of organizing such district. All drainage districts having valid corporate existence before July 1, 1972, shall enjoy all rights, duties, powers and authorities conferred by sections 31-401 to 31-450 and shall not be affected by this section, nor shall the legality of formation, organization, or operation of any such district be subject to any legal action based on this section.