The Catholic Church teaches that a heretic would cease to be pope, and that a heretic couldn’t be validly elected pope
The Catholic Encyclopedia, “Heresy,” 1914, Vol. 7, p. 261: “The pope
himself, if notoriously guilty of heresy, would cease to be pope
because he would cease to be a member of the Church.”

Pope Paul IV issued a Papal Bull (Cum ex Apostolatus Officio, Feb. 15, 1559) solemnly declaring that the election of a heretic as pope is null and void: "...that if ever at any time it shall appear that any Bishop, even if he be acting as an Archbishop, Patriarch or Primate; or any Cardinal of the aforesaid Roman Church, or, as has already been mentioned, any legate, or even the Roman Pontiff, prior to his promotion or his elevation as Cardinal or Roman Pontiff, has deviated from the Catholic Faith or fallen into some heresy:
(i) the promotion or elevation, even if it shall have been uncontested and by the unanimous assent of all the Cardinals, shall be null, void and worthless...

Father Nicholson:

Thank you for this courageous talk. May God be praised in His Holy Priesthood