Neb. Rev. Stat. § 44-4335

For a period of thirty days after such publication, any person in interest shall have the right to contest the legality of such resolution or proceeding or any bonds which may be authorized thereby, any provisions made for the security and payment of such bonds, any contract of purchase, sale, or lease, or any insurance policy contract, and after such time no one shall have any cause of action to contest the regularity, formality, or legality thereof for any cause whatsoever.