Flex - Binding ViewStack selectedChild Property Using a String Value

TypeError: Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/getChildIndex()
    at mx.core::Container/getChildIndex()[E:dev3.0.xframeworksprojectsframeworksrcmxcoreContainer.as:2409]
    at mx.containers::ViewStack/set selectedChild()[E:dev3.0.xframeworksprojectsframeworksrcmxcontainersViewStack.as:557]


<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
    <mx:Script>
        <![CDATA[
            [Bindable]
            private var targetViewName:String = "content";
        ]]>
    </mx:Script>

    <mx:ViewStack id="viewStack" width="100%" height="100%"
        selectedChild="{Container(viewStack.getChildByName(targetViewName))}">
        <mx:Panel id="welcome" width="100%" height="100%" />

        <mx:Panel id="content" width="100%" height="100%" />
    </mx:ViewStack>
</mx:Application>

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
    <mx:Script>
        <![CDATA[
            import mx.core.Container;
            [Bindable]
            private var targetViewName:String = "content";

            private function onClick() : void
            {
                viewStack.selectedChild = Container(viewStack.getChildByName(targetViewName)) ;
            }
        ]]>
    </mx:Script>

    <mx:ViewStack id="viewStack" width="100%" height="100%" >
        <mx:Panel id="welcome" width="100%" height="100%"  title="welcome"/>

        <mx:Panel id="content" width="100%" height="100%" title="content" />
    </mx:ViewStack>

    <mx:Button click="onClick()" label="click" />

</mx:Application>

selectedChild="{this[targetViewName]}">

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
    <mx:Script>
        <![CDATA[
            [Bindable]
            private var targetViewName:String = "content";
        ]]>
    </mx:Script>

    <mx:TabNavigator id="viewStack" width="100%" height="100%" creationPolicy="all"
        selectedChild="{this[targetViewName]}">
        <mx:Panel id="welcome" width="100%" height="100%" label="welcome" />

        <mx:Panel id="content" width="100%" height="100%" label="content" />
    </mx:TabNavigator>
</mx:Application>

<mx:TabNavigator id="viewStack" width="100%" height="100%" creationPolicy="all" >
    <mx:Panel id="welcome" width="100%" height="100%" label="welcome" />

    <mx:Panel id="content" width="100%" height="100%" label="content" addedToStage="viewStack.selectedChild = this" />
</mx:TabNavigator>

<mx:Script>
	<![CDATA[
		import models.ModelLocator;

		[Bindable]
		private var model:ModelLocator = ModelLocator.getInstance();
	]]>
</mx:Script>

<mx:ViewStack id="videoViewStack" width="100%" height="100%" selectedChild="{this[model._videoViewStack]}" >
	<viewsVideos:AllVideos id="AllVideos" label="Videos"/>
	<viewsVideos:MainVideo id="MainVideo" label="Video"/>
</mx:ViewStack>