Fix: VSE: Various crashes when sequencer scene is not initialized

Most of these crashes happen because it is assumed that the scene will
always be present even if we have an uninitialized `Editing`, which is
no longer the case with #140271.

- Fix crash when clicking and dragging in the scrub area by checking for
  valid `sequencer_scene` in `change_frame_poll`
- Fix crashes when selecting menu items by disabling them in the UI
  until a `sequencer_scene` is present
- Fix crashes running operators from the F3 menu by changing to more
  restrictive polls that check for `sequencer_scene`
- For good measure, check before dereferencing in
  `channels_displayed_get`, `active_seqbase_get`, and `editing_get`

Pull Request: https://projects.blender.org/blender/blender/pulls/145145

source/blender/editors/animation/anim_ops.cc

@@ -111,6 +111,9 @@ static bool change_frame_poll(bContext *C)
       return true;
     }
     if (area->spacetype == SPACE_SEQ) {
+      if (!CTX_data_sequencer_scene(C)) {
+        return false;
+      }
       /* Check the region type so tools (which are shared between preview/strip view)
        * don't conflict with actions which can have the same key bound (2D cursor for example). */
       const ARegion *region = CTX_wm_region(C);

source/blender/editors/animation/keyframing.cc

@@ -930,7 +930,7 @@ void ANIM_OT_keyframe_clear_vse(wmOperatorType *ot)
   ot->invoke = clear_anim_vse_invoke;
   ot->exec = clear_anim_vse_exec;
 
-  ot->poll = ED_operator_areaactive;
+  ot->poll = ED_operator_sequencer_scene_editable;
 
   /* flags */
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
@@ -1120,7 +1120,7 @@ void ANIM_OT_keyframe_delete_vse(wmOperatorType *ot)
   ot->invoke = delete_key_vse_invoke;
   ot->exec = delete_key_vse_exec;
 
-  ot->poll = ED_operator_areaactive;
+  ot->poll = ED_operator_sequencer_scene_editable;
 
   /* flags */
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;

source/blender/editors/sound/sound_ops.cc

@@ -39,6 +39,7 @@
 #include "RNA_prototypes.hh"
 
 #include "SEQ_iterator.hh"
+#include "SEQ_sequencer.hh"
 
 #include "UI_interface.hh"
 #include "UI_interface_layout.hh"
@@ -776,7 +777,7 @@ static void SOUND_OT_mixdown(wmOperatorType *ot)
 
 static bool sound_poll(bContext *C)
 {
-  Editing *ed = CTX_data_sequencer_scene(C)->ed;
+  Editing *ed = blender::seq::editing_get(CTX_data_sequencer_scene(C));
 
   if (!ed || !ed->act_strip || ed->act_strip->type != STRIP_TYPE_SOUND_RAM) {
     return false;
@@ -789,7 +790,7 @@ static bool sound_poll(bContext *C)
 static wmOperatorStatus sound_pack_exec(bContext *C, wmOperator *op)
 {
   Main *bmain = CTX_data_main(C);
-  Editing *ed = CTX_data_sequencer_scene(C)->ed;
+  Editing *ed = blender::seq::editing_get(CTX_data_sequencer_scene(C));
   bSound *sound;
 
   if (!ed || !ed->act_strip || ed->act_strip->type != STRIP_TYPE_SOUND_RAM) {
@@ -862,7 +863,7 @@ static wmOperatorStatus sound_unpack_exec(bContext *C, wmOperator *op)
 
 static wmOperatorStatus sound_unpack_invoke(bContext *C, wmOperator *op, const wmEvent * /*event*/)
 {
-  Editing *ed = CTX_data_sequencer_scene(C)->ed;
+  Editing *ed = blender::seq::editing_get(CTX_data_sequencer_scene(C));
   bSound *sound;
 
   if (RNA_struct_property_is_set(op->ptr, "id")) {

source/blender/editors/space_sequencer/sequencer_proxy.cc

@@ -141,6 +141,7 @@ void SEQUENCER_OT_rebuild_proxy(wmOperatorType *ot)
   /* API callbacks. */
   ot->invoke = sequencer_rebuild_proxy_invoke;
   ot->exec = sequencer_rebuild_proxy_exec;
+  ot->poll = sequencer_edit_poll;
 
   /* Flags. */
   ot->flag = OPTYPE_REGISTER;
@@ -236,6 +237,7 @@ void SEQUENCER_OT_enable_proxies(wmOperatorType *ot)
   /* API callbacks. */
   ot->invoke = sequencer_enable_proxies_invoke;
   ot->exec = sequencer_enable_proxies_exec;
+  ot->poll = sequencer_edit_poll;
 
   /* Flags. */
   ot->flag = OPTYPE_REGISTER;

source/blender/editors/space_sequencer/sequencer_view.cc

@@ -280,7 +280,8 @@ void SEQUENCER_OT_view_all_preview(wmOperatorType *ot)
 
 static wmOperatorStatus sequencer_view_zoom_ratio_exec(bContext *C, wmOperator *op)
 {
-  const RenderData *rd = &CTX_data_sequencer_scene(C)->r;
+  const Scene *scene = CTX_data_sequencer_scene(C);
+  const RenderData *rd = &scene->r;
   View2D *v2d = UI_view2d_fromcontext(C);
 
   float ratio = RNA_float_get(op->ptr, "ratio");

source/blender/sequencer/intern/channels.cc

@@ -27,7 +27,7 @@ namespace blender::seq {
 
 ListBase *channels_displayed_get(const Editing *ed)
 {
-  return ed->current_channels();
+  return ed ? ed->current_channels() : nullptr;
 }
 
 void channels_displayed_set(Editing *ed, ListBase *channels)

source/blender/sequencer/intern/sequencer.cc

@@ -274,7 +274,7 @@ void seq_free_strip_recurse(Scene *scene, Strip *strip, const bool do_id_user)
 
 Editing *editing_get(const Scene *scene)
 {
-  return scene->ed;
+  return scene ? scene->ed : nullptr;
 }
 
 Editing *editing_ensure(Scene *scene)
@@ -422,11 +422,7 @@ int tool_settings_pivot_point_get(Scene *scene)
 
 ListBase *active_seqbase_get(const Editing *ed)
 {
-  if (ed == nullptr) {
-    return nullptr;
-  }
-
-  return ed->current_strips();
+  return ed ? ed->current_strips() : nullptr;
 }
 
 void active_seqbase_set(Editing *ed, ListBase *seqbase)