Refactor: convert "Animated Transforms to Deltas" to current Action API

Convert the code for the "Animated Transforms to Deltas" 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/object.py

@@ -14,6 +14,7 @@ from bpy.app.translations import (
     pgettext_rpt as rpt_,
     contexts as i18n_contexts,
 )
+from bpy_extras import anim_utils
 
 
 class SelectPattern(Operator):
@@ -843,7 +844,10 @@ class TransformsToDeltasAnim(Operator):
             # first pass over F-Curves: ensure that we don't have conflicting
             # transforms already (e.g. if this was applied already) #29110.
             existingFCurves = {}
-            for fcu in adt.action.fcurves:
+            channelbag = anim_utils.action_get_channelbag_for_slot(adt.action, adt.action_slot)
+            if not channelbag:
+                continue
+            for fcu in channelbag.fcurves:
                 # get "delta" path - i.e. the final paths which may clash
                 path = fcu.data_path
                 if path in STANDARD_TO_DELTA_PATHS:
@@ -878,7 +882,7 @@ class TransformsToDeltasAnim(Operator):
 
             # if F-Curve uses standard transform path
             # just append "delta_" to this path
-            for fcu in adt.action.fcurves:
+            for fcu in channelbag.fcurves:
                 if fcu.data_path == "location":
                     fcu.data_path = "delta_location"
                     obj.location.zero()