Exception when adding a spark RadioButton at runtime

Steps to reproduce:
1. I have not been able to create a standalone case, but the RTE happens every time in Catalyst using the steps outlined in bug TH-4332.
The stack trace from the RTE is:

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
at flash.display::DisplayObjectContainer/getChildIndex()
at mx.core::Container/http://www.adobe.com/2006/flex/mx/internal::rawChildren_getChildIndex()[E:\dev\trunk\frameworks\projects\framework\src\mx\core\Container.as:4437] <http://www.adobe.com/2006/flex/mx/internal::rawChildren_getChildIndex()%5bE:/dev/trunk/frameworks/projects/framework/src/mx/core/Container.as:4437%5d> <http://www.adobe.com/2006/flex/mx/internal::rawChildren_getChildIndex()%5bE:/dev/trunk/frameworks/projects/framework/src/mx/core/Container.as:4437%5d>
at mx.core::ContainerRawChildrenList/getChildIndex()[E:\dev\trunk\frameworks\projects\framework\src\mx\core\ContainerRawChildrenList.as:189]
at spark.components::RadioButtonGroup/breadthOrderCompare()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButtonGroup.as:589]
at spark.components::RadioButtonGroup/breadthOrderCompare()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButtonGroup.as:603]
at spark.components::RadioButtonGroup/breadthOrderCompare()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButtonGroup.as:603]
at spark.components::RadioButtonGroup/breadthOrderCompare()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButtonGroup.as:603]
at Array$/_sort()
at Array/http://adobe.com/AS3/2006/builtin::sort()
at spark.components::RadioButtonGroup/http://www.adobe.com/2006/flex/mx/internal::addInstance()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButtonGroup.as:424] <http://www.adobe.com/2006/flex/mx/internal::addInstance()%5bE:/dev/trunk/frameworks/projects/flex4/src/spark/components/RadioButtonGroup.as:424%5d> <http://www.adobe.com/2006/flex/mx/internal::addInstance()%5bE:/dev/trunk/frameworks/projects/flex4/src/spark/components/RadioButtonGroup.as:424%5d>
at spark.components::RadioButton/addToGroup()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButton.as:454]
at spark.components::RadioButton/commitProperties()[E:\dev\trunk\frameworks\projects\flex4\src\spark\components\RadioButton.as:407]
at mx.core::UIComponent/validateProperties()[E:\dev\trunk\frameworks\projects\framework\src\mx\core\UIComponent.as:7019]
at mx.managers::LayoutManager/validateProperties()[/Users/greg/Thermo/Depot_alt/flex/ide_common/ArtBoard/src/mx/managers/LayoutManager.as:567]
at mx.managers::LayoutManager/doPhasedInstantiation()[/Users/greg/Thermo/Depot_alt/flex/ide_common/ArtBoard/src/mx/managers/LayoutManager.as:731]