Fix: Whole character keying set error on unresolved paths

The "Whole Character" keying set might fail to insert keys
when a property cannot be resolved.
This occurred on the Ellie rig from the Blender Studio.
The failing property was called "bone gizmo".
The fix is to check if `rna_property` exists.

Pull Request: https://projects.blender.org/blender/blender/pulls/122038

scripts/startup/keyingsets_builtins.py

@@ -519,6 +519,9 @@ class WholeCharacterMixin:
                     # This happens when a custom property is set to None. In that case it cannot
                     # be converted to an FCurve-compatible value, so we can't keyframe it anyway.
                     continue
+                if not rna_property:
+                    # Failure to resolve property.
+                    continue
                 if rna_property.rna_type in prop_type_compat:
                     self.addProp(ks, bone, prop_path)
             elif prop_rna.is_animatable: