Fix undo crash with multiple scenes & windows

Creating a new window then a new scene would crash on undo
because the undo step loaded would not contain the new windows scene.

Regression introduced since 3.6.

source/blender/blenkernel/intern/blendfile.cc

@@ -800,6 +800,19 @@ static void setup_app_data(bContext *C,
     wm_data_consistency_ensure(CTX_wm_manager(C), curscene, cur_view_layer);
   }
 
+  if (mode == LOAD_UNDO) {
+    /* It's possible to undo into a time before the scene existed, in this case the window's scene
+     * will be null. Since it doesn't make sense to remove the window, set it to the current scene.
+     * NOTE: Redo will restore the active scene to the window so a reasonably consistent state
+     * is maintained. We could do better by keeping a window/scene map for each undo step. */
+    wmWindowManager *wm = static_cast<wmWindowManager *>(bfd->main->wm.first);
+    LISTBASE_FOREACH (wmWindow *, win, &wm->windows) {
+      if (win->scene == nullptr) {
+        win->scene = curscene;
+      }
+    }
+  }
+
   BLI_assert(BKE_main_namemap_validate(bfd->main));
 
   if (mode != LOAD_UI) {