Refactor: convert rigid body "Bake to Keyframes" to current Action API

Convert the code for the rigid body "Bake to Keyframes" operator from the
legacy Actions API to the current API.

No functional changes.

This is part of #146586

Pull Request: https://projects.blender.org/blender/blender/pulls/147060

scripts/startup/bl_operators/rigidbody.py

@@ -8,6 +8,7 @@ from bpy.props import (
     EnumProperty,
     IntProperty,
 )
+from bpy_extras import anim_utils
 
 
 class CopyRigidbodySettings(Operator):
@@ -167,8 +168,13 @@ class BakeToKeyframes(Operator):
 
             # clean up keyframes
             for obj in objects:
-                action = obj.animation_data.action
-                for fcu in action.fcurves:
+                channelbag = anim_utils.action_get_channelbag_for_slot(
+                    obj.animation_data.action,
+                    obj.animation_data.action_slot,
+                )
+                if not channelbag:
+                    continue
+                for fcu in channelbag.fcurves:
                     keyframe_points = fcu.keyframe_points
                     i = 1
                     # remove unneeded keyframes