Section 21. Judgments of divorce shall in the first instance be judgments nisi, and shall become absolute after the expiration of ninety days from the entry thereof, unless the court within said period, for sufficient cause, upon application of any party to the action, otherwise orders. After the entry of a judgment nisi, the action shall not be dismissed or discontinued on motion of either party except upon such terms, if any, as the court may order after notice to the other party and a hearing, unless there has been filed with the court a memorandum signed by both parties, wherein they agree to such disposition of the action.