Hi folks, I already created a JIRA issue for this: https://issues.apache.org/jira/browse/FLEX-34390
[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 ) { queryInput.text = "Hello World"; } } 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] DarkStone 2014-07-01