Check the side window configuration of FRAME.
FRAME defaults to the selected frame.
A valid side window configuration preserves the following two
invariants:
- If there exists a window whose window-side parameter is
non-nil, there must exist at least one live window whose
window-side parameter is nil.
- If a window W has a non-nil window-side parameter (i) it must
have a parent window and that parent's window-side parameter
must be either nil or the same as for W, and (ii) any child
window of W must have the same window-side parameter as W.
If the configuration is invalid, reset the window-side parameters
of all windows on FRAME to nil.