This fixes a crash when SSL's channelstrip calls set-selection
early on during session load, during Session::load_routes.
Session::add_routes_inner calls calls ARDOUR::GUIIdle() which
can trigger a SSL Native Channel Strip 2" VST3 to call
VST3PI::setContextInfoValue, which in turn emits a
CoreSelection::send_selection_change before the session is
fully loaded.
This also handles various edge cases where a given AC may
not [yet] exist.
e34359b921