145A.22  Actions subject to contest of elections — filing actions — limitation.
  A special election called to approve or reject an original plan of merger or an amendment to an approved plan of merger is subject to the provisions for contest of elections for public measures set forth in chapter 57. Except as provided with respect to election contests, after one hundred twenty days following the third and final publication of the order of approval of the plan or amendment to the plan of merger, an action shall not be filed to contest the regularity of the proceedings with respect to a plan of merger or amendment to a plan of merger. After one hundred twenty days the organization of the merged area is conclusively presumed to have been lawful.
85 Acts, ch 123, §12