No informality in any proceeding or in the conduct of any election, not substantially affecting adversely the legal rights of any citizen, shall be held to invalidate the incorporation of any district. Any proceedings wherein the validity of incorporation is sought to be denied shall be commenced within 20 days after the date of the certificate of incorporation, otherwise the incorporation and the legal existence of the district, and all proceedings in respect thereto, shall be held valid and in every respect legal and incontestable.