Section 12. No action commenced under section six shall be dismissed, except upon a sworn statement made and filed by the plaintiff and by his attorney setting forth the reasons for dismissal thereof and upon approval of such dismissal by the court in open court. If the court is of opinion that the action ought not to be dismissed he may direct the district attorney to prosecute the case to judgment. If the action was brought by a citizen and the court finds that there was no reasonable ground therefor, costs may be awarded against the plaintiff.