The new Action selector (51fd355c01) avoids the need to use
`space_data.action` in the dope sheet (that attribute was a bit of a
hack; the new Action selector was made to avoid that). It failed to take
the dope sheet's shape key mode into account properly, though. This is
now fixed.
Pull Request: https://projects.blender.org/blender/blender/pulls/127177