No informality in any proceeding or in the conduct of the election, not substantially affecting adversely the legal rights of any citizen, shall be held to invalidate the incorporation of the district. Any proceedings wherein the validity of incorporation is denied shall be commenced within three months from the date of filing the order declaring the result of the election with the Secretary of State, otherwise the incorporation and the legal existence of the district shall be held to be valid and in every respect legal and incontestable.