> I don't know if the author of
 > https://www.reddit.com/r/emacs/comments/sthusa/window_xxx_is_not_a_valid_window_on_switching_tabs/
 > will report the bug here,

Please encourage the author again to report such bugs here.

 > but since it's easily reproducible,
 > here is the short test case, just evaluate this:
 >
 > (window-state-put
 > ...
 >   nil 'safe)
 >
 > It fails with:
 >
 >    Debugger entered--Lisp error: (error "#<window 9> is not a valid window")
 >      error("%s is not a valid window" #<window 9>)
 >      window-normalize-window(#<window 9>)
 >      window-deletable-p(#<window 9>)
 >      window-state-put

I think the attached fixes it, please check.  I have no idea whether the
state is restored as intended, though.

Thanks, martin