Fix #147577: Runtime Error warning on loading default workspace

Caused by null de-reference when accessing `sequencer_scene`.

Return false if sequencer scene is null to indicate, that the path to
strips or modifiers can not be created.

Pull Request: https://projects.blender.org/blender/blender/pulls/147594

source/blender/editors/space_buttons/buttons_context.cc

@@ -615,6 +615,9 @@ static bool buttons_context_path(
   /* No pinned root, use scene as initial root. */
   else if (mainb != BCONTEXT_TOOL) {
     if (ELEM(mainb, BCONTEXT_STRIP, BCONTEXT_STRIP_MODIFIER)) {
+      if (!sequencer_scene) {
+        return false;
+      }
       path->ptr[0] = RNA_id_pointer_create(&sequencer_scene->id);
     }
     else {