What is the ruling on the prayer of a person praying alone after he intended to lead the prayer and the Imam's ablution was nullified during the prayer, knowing that the Imam was the one who called the iqamah for the prayer?

Scholars differ on the invalidation of the prayer of the follower if the Imam becomes aware of a ritual impurity during the prayer. The relied-upon view among the Hanbalis is that it invalidates [the prayer]. While others believe that the follower's prayer does not invalidate; rather, they complete it and it is sound. Ibn Qasim mentioned that the prayer of the follower is valid if both he and the Imam were unaware of the ritual impurity until the prayer concluded; otherwise, they should restart. Sheikh Ibn Uthaymin preferred the view that if the Imam remembers his ritual impurity during the prayer, he should appoint someone to lead the congregants in prayer, or the congregants should put forward one of them, or they can complete their prayers individually, and their prayers are sound due to the absence of evidence for their invalidation.