This crashed because in `get_active_fcurve_channel`, the filter did not filter out channels with no fcurve. The fix adds the filter `ANIMFILTER_FCURVESONLY`. See rB92d7f9ac56e0ff1e65c364487542dfb7c32a0a67 for the new filter. Maniphest Tasks: T99491 Differential Revision: https://developer.blender.org/D15386