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
7bd19f7efb