bug#28978: 26.0; Regression: separate, dedicated `*Completions*' frame no longer has parameter `minibuffer'

[martin rudalics]

> It should be the case in my context that this returns the minibuffer
> window:
>
> (cdr (assoc 'minibuffer (frame-parameters
>                            (#<frame *Completions* 000000000cacf6c8>))))
>
> Instead, it now returns nil.  Parameter `minibuffer' for the
> `*Completions*' frame is no longer the minibuffer window; it is nil.

Since the 'minibuffer' parameter of a frame cannot be changed (from nil
to some non-nil value and vice-versa at least) after its creation, your
code somehow now seems to fail to set up that parameter.  Please show
the corresponding part where your code tries to set the parameter.  If
necessary, debug it to show that after the corresponding ‘make-frame’
call the frame does not have that parameter set as requested.

Thanks, martin