Don't try to reset state if last window is undefined

`old-selected-window` may return a frame instead of a window, or `nil` if there
is not a previous window selection (e.g. at Emacs startup). In either of these
cases, trying to find the old buffer and reset its state would produce an error,
so do nothing instead.

boon-core.el

@@ -373,9 +373,10 @@ the buffer changes."
 ;; surprising to the user when coming back to it).
 (defun boon-reset-state-for-switchw (new-frame)
   "Reset the boon state to natural when switching windows."
-  (let* ((old (old-selected-window))
-         (prev-buf (window-buffer old)))
-    (with-current-buffer prev-buf
+  (-when-let* ((old-frame-or-window (old-selected-window))  ; `old-selected-window' sometimes (surprisingly) returns a frame.
+               (old-window (and (windowp old-frame-or-window) old-frame-or-window))
+               (old-buffer (window-buffer old-window)))
+    (with-current-buffer old-buffer
       (boon-set-natural-state))))
 
 (add-hook 'window-selection-change-functions #'boon-reset-state-for-switchw)