from emacs -Q:

set up three frames, a, b and c:

    (set-frame-parameter c 'parent-frame a)

c becomes child of a.

(set-frame-parameter c 'parent-frame b)

expected: c becomes child of b and is no longer child of a.

actual: c remains child of a, and b now cannot be raised above a and b.

(set-frame-parameter c 'parent-frame nil)

expected: c ceases to be a child frame.

actual: emacs crashes.



Patch attached.

If this is too late for 26.1 I don’t think that will be a huge issue
as it seems redefining parent-frame is not something people want to
do, and it’s unlikely to be something a user does by accident.
-- 
Alan Third