Normally, the dope sheet will not show the directly-assigned Action,
when that Action has no animation data for the data-block.
However, when that data-block uses the NLA, and one or more NLA strips
have an animated property (like their influence), the Action is shown in
the dope sheet for some reason.
This caused the "draw the summary line for this Action" to fail, because
it assumes there must be a slot (because normally when it's drawn, it
implies there is animation data, and thus a slot).
This assumption has been fixed, and the "no slot" case is now handled
properly.
Pull Request: https://projects.blender.org/blender/blender/pulls/137844
b59b4d87a8