Having “on_all_workspaces_requested” FALSE on a window does not imply a
workspace is set.

If the X11 window is placed on a secondary monitor while workspaces
applies on primary monitor only  (“workspaces-only-on-primary” set) then
“on_all_workspaces_requested” is FALSE while “on_all_workspaces“ is TRUE
and the associated workspace is NULL, leading to a crash when saving the
gnome-shell/mutter session.

So if no workspace is set, use the “initial_workspace” instead to avoid
a NULL pointer dereference.

https://bugzilla.gnome.org/show_bug.cgi?id=792818