Fix #135582: Crash box selecting in sequence editor without a sequence

Caused by f98b01e492

The issue of uninitialized `Editing` (in new scenes) has come up before,
see e.g. https://projects.blender.org/blender/blender/issues/
118633#issuecomment-1134447

So it is not uncommon to guard against `Editing` being null, same is now
done here

Pull Request: https://projects.blender.org/blender/blender/pulls/135623

source/blender/editors/space_sequencer/sequencer_select.cc

@@ -996,6 +996,10 @@ static blender::Vector<Strip *> mouseover_strips_sorted_get(const Scene *scene,
 {
   Editing *ed = SEQ_editing_get(scene);
 
+  if (ed == nullptr) {
+    return {};
+  }
+
   blender::Vector<Strip *> strips;
   LISTBASE_FOREACH (Strip *, strip, ed->seqbasep) {
     if (strip->machine != int(mouse_co[1])) {