Required skin part area cannot be found.

Problem:

Error: Required skin part area cannot be found.
    at spark.components.supportClasses::SkinnableComponent/findSkinParts()[C:\flexsource\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:549]
    at spark.components.supportClasses::SkinnableComponent/loadSkin()[C:\flexsource\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:524]
    at spark.components.supportClasses::SkinnableComponent/validateSkinChange()[C:\flexsource\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:280]
    at spark.components.supportClasses::SkinnableComponent/createChildren()[C:\flexsource\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:250]
    at com.riageeks.geek::GeekBase/createChildren()[C:\geek\GeekBase.as:130]
    at mx.core::UIComponent/initialize()[C:\flexsource\frameworks\projects\framework\src\mx\core\UIComponent.as:6510]
    at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\flexsource\frameworks\projects\framework\src\mx\core\UIComponent.as:6402]
    at mx.core::UIComponent/addChildAt()[C:\flexsource\frameworks\projects\framework\src\mx\core\UIComponent.as:6109]
    at spark.components::Group/addItemToDisplayList()[C:\flexsource\frameworks\projects\flex4\src\spark\components\Group.as:1588]
    at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[C:\flexsource\frameworks\projects\flex4\src\spark\components\Group.as:1153]
    at spark.components::Group/setMXMLContent()[C:\flexsource\frameworks\projects\flex4\src\spark\components\Group.as:409]
    at spark.components::Group/set mxmlContent()[C:\flexsource\frameworks\projects\flex4\src\spark\components\Group.as:359]
    at spark.components::SkinnableContainer/set mxmlContent()[C:\flexsource\frameworks\projects\flex4\src\spark\components\SkinnableContainer.as:563]
    at spark.components::SkinnableContainer/createDeferredContent()[C:\flexsource\frameworks\projects\flex4\src\spark\components\SkinnableContainer.as:1084]
    at spark.components::SkinnableContainer/createContentIfNeeded()[C:\flexsource\frameworks\projects\flex4\src\spark\components\SkinnableContainer.as:1098]
    at spark.components::SkinnableContainer/createChildren()[C:\flexsource\frameworks\projects\flex4\src\spark\components\SkinnableContainer.as:843]
    at mx.core::UIComponent/initialize()[C:\flexsource\frameworks\projects\framework\src\mx\core\UIComponent.as:6510]
    at spark.components::Application/initialize()[C:\flexsource\frameworks\projects\flex4\src\spark\components\Application.as:708]
    at thingsapp/initialize()
    at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\flexsource\frameworks\projects\framework\src\mx\managers\SystemManager.as:2263]
    at mx.managers::SystemManager/initializeTopLevelWindow()[C:\flexsource\frameworks\projects\framework\src\mx\managers\SystemManager.as:3610]
    at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::docFrameHandler()[C:\flexsource\frameworks\projects\framework\src\mx\managers\SystemManager.as:3395]
    at mx.managers::SystemManager/docFrameListener()[C:\flexsource\frameworks\projects\framework\src\mx\managers\SystemManager.as:3253]

Solution: In my case, I forgot to define a SkinPart for a component, you manually have to go through each Skin file and then verify which skin you missed in your component.