st: Be more forgiving when calling get_theme_node() on unstaged widgets

While it is obviously still an error to call get_theme_node() on a
widget that hasn't been added to the stage hierarchy yet, asserting
on it hasn't proven too successful in avoiding those errors - it's
likely the most frequent reason for crash reports. Just accept that
there'll always be code paths where we can hit this case and make
it non-fatal.

https://bugzilla.gnome.org/show_bug.cgi?id=610279