Exception setting mask of RichText to null

Steps to reproduce:
1. Run the following program and click the "Set Null Mask" button

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600">
<fx:Script>
<![CDATA[
public function setNullMask():void

]]
>
</fx:Script>
<s:RichText id="rich" x="10" y="60" text="RichText">
<s:mask>
<s:Group>
</s:Group>
</s:mask>
</s:RichText>
<s:Button x="10" y="10" label="Set Null Mask" click="setNullMask()"/>
</s:Application>

Actual Results:
Mask does get removed visually, but get an exception

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at spark.components::RichText/commitProperties()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\RichText.as:1057]
at mx.core::UIComponent/validateProperties()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7772]
at mx.managers::LayoutManager/validateProperties()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\LayoutManager.as:572]
at mx.managers::LayoutManager/doPhasedInstantiation()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\LayoutManager.as:730]
at mx.managers::LayoutManager/doPhasedInstantiationCallback()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\LayoutManager.as:1072]

Expected Results:
Mask is removed and no exception

Workaround (if any):