Vacancy or defect not to invalidate proceedings.

No act or proceeding of the Board under this Act shall be questioned on the ground merely of the existence of any vacancy in, or defect in the appointment of a member or in the constitution of the Board: Provided that the Board shall not act or take any proceeding at any time when, by reason of any vacancy occurring, the number of continuing non-official members is less than one-half of the total number of such nonofficial members in the Board.