Vacancies etc., not to invalidate proceedings of Board

No act done, or proceeding taken, under this Act by the Board shall be invalid merely on the ground,- (a) of any vacancy or defect in the constitution of the Board; or (b) of any defect or irregularity in the election or appointment of a person acting as a member thereof; or (c) of any defect or irregularity in such act or proceeding, not affecting the merits of the case.