Problem : Some of us encounter the Flex unit runtime issue when running the flex unit in the flash builder a few days ago, and it blocked our running the unit test without the ant script. Error: Skin for FlexUnitApplication cannot be found. at spark.components.supportClasses::SkinnableComponent/attachSkin()[E:/dev/4.0.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:631] at spark.components.supportClasses::SkinnableComponent/validateSkinChange()[E:/dev/4.0.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:404] at spark.components.supportClasses::SkinnableComponent/createChildren()[E:/dev/4.0.0/frameworks/projects/spark/src/spark/components/supportClasses/SkinnableComponent.as:367] at spark.components::SkinnableContainer/createChildren()[E:/dev/4.0.0/frameworks/projects/spark/src/spark/components/SkinnableContainer.as:821] at mx.core::UIComponent/initialize()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/core/UIComponent.as:7250] at spark.components::Application/initialize()[E:/dev/4.0.0/frameworks/projects/spark/src/spark/components/Application.as:916] at FlexUnitApplication/initialize() at mx.managers.systemClasses::ChildManager/childAdded()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/managers/systemClasses/ChildManager.as:189] at mx.managers.systemClasses::ChildManager/initializeTopLevelWindow()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/managers/systemClasses/ChildManager.as:341] at mx.managers::SystemManager/initializeTopLevelWindow()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/managers/SystemManager.as:2810] at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::kickOff()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/managers/SystemManager.as:2637] at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::preloader_completeHandler()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/managers/SystemManager.as:2539] at flash.events::EventDispatcher/dispatchEventFunction() at flash.events::EventDispatcher/dispatchEvent() at mx.preloaders::Preloader/timerHandler()[E:/dev/4.0.0/frameworks/projects/framework/src/mx/preloaders/Preloader.as:515] at flash.utils::Timer/_timerDispatch() at flash.utils::Timer/tick()
Summary
: It is a issue of the Flex Unit framework, not relative to our test cases. I guess it was caused by the Flash Player version upgrade.
Solution
: If you encounter this exception again, you can modify the file FlexUnitApplication.mxml manually as following (which should be created by flex unit and not need to modify before)