The judge on the stand?
Properly Done: A witness must be listed or motioned for, and witnesses can be denied
Any party in a lawsuit needs to list all witnesses well in advance of the trial. Presenting the witness list is part of normal motion practice. The law is such that there should be no surprise witnesses because both parties need to be able to prepare for the witnesses.
If a new witness or evidence surfaces during the trial, there's a motion to present them. That motion can be oral, during the trial. But without such a motion, they can't be heard.
As such, the judge will review the request days to months in advance and deny the motion. The lawyer could try to appeal that, but unless he has extremely good reasons to get the judge into the witness stand, then the motion will be denied, because the judge is extremely rarely a relevant witness.
When the lawyer actually had such good reasons, then the judge would have needed to recuse themselves in the first place, and can't preside over the case.
In the heat of the moment
If the lawyer has the glorious idea to request the judge to take the stand during the trial, the Judge can do a lot of things, starting with denying the motion. If the lawyer does not stop, then the judge can order the lawyer to shut up, then fine them for contempt of court, and finally lock them up for contempt of court.
The proper way to deal with the ad-hoc motion being denied is to appeal the decision, which does not lead to a mistrial and if the denial was proper, nothing at all.
Who's a witness?
Only relevant people may be called as witnesses. Relevant means, that their testimony will lead to helping the case to progress.
Unless the sweet old lady that has to tell lots of good things about the accused actually has something case relevant to say, she is not a witness that can be called for anything but a character witness.
The President and pope would almost never be relevant witnesses, and so is the judge.
The judge can smack down any nonrelevant witness, and deny that they can be heard at all. It is up to the person calling the witness to establish that and how they are relevant, when they provide their list of witnesses to the court.