SkinnableTextBase bug (found in Flex 4.13.0 RC2)

[Short story]

There is a bug in SkinnableTextBase, inside the function body of "private function textDisplay_changeHandler(event:Event):void", the skin.currentState is null when running on mobile, I don't know how to fix it, but at least I know we should add a null check to the skin.currentState.

Please see the screenshot I attached:
https://issues.apache.org/jira/secure/attachment/12653403/SkinnableTextBase%20bug%20%28found%20in%20Flex%204.13.0%20RC2%29.png

[Steps to reproduce the bug]

1. Create a subclass of SkinnableComponent, name the subclass SimpleTest.

2. Define a SkinPart of SimpleTest:

[SkinPart(required="true")]
public var queryInput:TextInput;

3. override partAdded function for SimpleTest:

override protected function partAdded(partName:String, instance:Object):void
{
if ( instance == queryInput )

}

4. Create the SimpleTestSkin.mxml for SimpleTest, use the <s:TextInput id="queryInput"/> to bind the SkinPart, use the TextInput's default skin.

5. Bind the SimpleTestSkin.mxml to SimpleTest using CSS.
s|SkinnableComponent.simpleTest
{
skinClass: ClassReference("skin.SimpleTestSkin");
}

6. Put <ns:SimpleTest styleName="simpleTest"/> inside <s:Application>

7. Run the application on Web or Desktop, no problems.

8. Run the application on AIR Simulator for mobile or run it on real mobile devices, will encounter the following errors:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at spark.components.supportClasses::SkinnableTextBase/textDisplay_changeHandler()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableTextBase.as:2703]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:13682]
at spark.components.supportClasses::ScrollableStageText/set text()[/Users/aharui/release4.13.0/frameworks/projects/mobilecomponents/src/spark/components/supportClasses/ScrollableStageText.as:1016]
at spark.components.supportClasses::SkinnableTextBase/set text()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableTextBase.as:1410]
at spark.components::TextInput/set text()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/TextInput.as:288]
at global/SYSTEM.LIB.display.updaters::updateTextComponentByCharacterAndOS()[D:\VLP\FLEX\NewsCenterDesktop\src\SYSTEM\LIB\display\updaters\updateTextComponentByCharacterAndOS.as:41]
at SYSTEM.COM.panes::FeedBasicEditorPane/partAdded()[D:\VLP\FLEX\NewsCenterDesktop\src\SYSTEM\COM\panes\FeedBasicEditorPane.as:119]
at SYSTEM.COM.panes::FeedEssentialEditorPane/partAdded()[D:\VLP\FLEX\NewsCenterDesktop\src\SYSTEM\COM\panes\FeedEssentialEditorPane.as:112]
at SYSTEM.COM.panes::SiteEditorPane/partAdded()[D:\VLP\FLEX\NewsCenterDesktop\src\SYSTEM\COM\panes\SiteEditorPane.as:65]
at spark.components.supportClasses::SkinnableComponent/findSkinParts()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:751]
at spark.components.supportClasses::SkinnableComponent/attachSkin()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:716]
at spark.components.supportClasses::SkinnableComponent/validateSkinChange()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:457]
at spark.components.supportClasses::SkinnableComponent/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:416]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1707]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1707]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1707]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at SYSTEM.SKIN.panes::SubscriptionEditorPaneSkin/initialize()
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChild()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7237]
at spark.components.supportClasses::SkinnableComponent/attachSkin()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:707]
at spark.components.supportClasses::SkinnableComponent/validateSkinChange()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:457]
at spark.components.supportClasses::SkinnableComponent/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:416]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1703]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1707]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]
at mx.core::UIComponent/initialize()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7695]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7556]
at mx.core::UIComponent/addChildAt()[/Users/aharui/release4.13.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7260]
at spark.components::Group/addDisplayObjectToDisplayList()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:2116]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:1707]
at spark.components::Group/setMXMLContent()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:653]
at spark.components::Group/createChildren()[/Users/aharui/release4.13.0/frameworks/projects/spark/src/spark/components/Group.as:906]