I'm not a lawyer, but I know that it's a bad idea to take legal advice from your adversary.
Mike Johnson wants Democratic voters to think that there's some issue with running anyone but Joe Biden. There isn't. The convention hasn't happened yet. The Heritage Foundation has written a memo about lawsuits but who are they going to sue? The delegates pledged to Biden-Harris for voting for Kamala Harris? I think Marc Elias, a lawyer for the Democrats, has this one covered.