Caused by recent refactor of strip input querying in #136474. This caused `Strip::seq1` to be set even if effect strip has no inputs. Fix was applied to `vse::strip_effect_get_new_inputs`, so it is less confusing during debugging, but also to `seq::add_effect_strip`, as I think, that core function should never produce invalid strip. Pull Request: https://projects.blender.org/blender/blender/pulls/138016