In case a scene strips `Input` was set to `Sequencer`, but the scene did
not actually have `Editing` data (e.g. when just adding a `New` scene as
scene strip), blender would crash trying to render.
Now simply check if the required `Editing` data is present before
proceeding.
Pull Request: https://projects.blender.org/blender/blender/pulls/120418
1fdd7cd098