It is not possible for the court to stay the proceeding of Section 125 crpc because the husband has filed a civil suit for the nullity of marriage. It is an undisputed fact that she is still your legally wedded wife. It is settled law that a legally wedded wife is entitled for maintenance under Section 125 crpc.
You have filed a suit under Section 12 of the Hindu Marriage Act 1955. The court has admitted your case and issued notice to your wife. This case is still pending.
So, during the pendency of the suit filed for nullity of marriage the court cannot stay the maintenance case filed under Section 125 crpc. Husband is bound to maintain his wife if he is able and has the capacity to earn.
In this situation your wife is also entitled to receive interim maintenance under Section 24 of the Hindu Marriage Act 1955 if she moves an application therein. Conclusion is that the court cannot stay the maintenance case filed under Section 125 crpc.