<fx:Script>   <![CDATA[     [Bindable] var RBlabel:String;            // variable used to capture the current (identified) layer name    [Bindable] var RBlayer:String;           //variable used to tell which layer to toggle on/off    [Bindable] var RBonsel:Boolean = false;  // variable used to capture "selected" state of the radio button    [Bindable] var RBoffsel:Boolean = true;  // variable used to capture "selected" state of the radio button     private function skin_preinitializeHandler(event:FlexEvent):void    {     // set the link styles //My Add                      off.visible = false;     on.visible = false;     RBlayer = "";     RBlabel = "";          if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/Buildings/MapServer/3"){      off.visible = true;      on.visible = true;          RBlayer = "Buildings";      RBlabel = "Building Labels";     }     if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/All_Waste_Sites/MapServer/1"){      off.visible = true;      on.visible = true;          RBlayer = "WasteSites";      RBlabel = "Point Labels";     }     if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/All_Waste_Sites/MapServer/2"){      off.visible = true;      on.visible = true;          RBlayer = "WasteSites";      RBlabel = "Line Labels";     }     if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/All_Waste_Sites/MapServer/3"){      off.visible = true;      on.visible = true;          RBlayer = "WasteSites";      RBlabel = "Poly Labels";     } //End My add     textLayoutConfiguration = new Configuration();     var textLayoutFormat:TextLayoutFormat = new TextLayoutFormat();     textLayoutFormat.color = getStyle("linkActiveColor")     textLayoutFormat.textDecoration = TextDecoration.UNDERLINE;     textLayoutConfiguration.defaultLinkActiveFormat = textLayoutFormat;      vGroup.addElement(ToggleLayers);   // My Add - this is not where I have it, just wanted to include it somewhere.   //My Add    protected function layerRBgroup_changeHandler(event:Event):void    {     var map:Map = hostComponent.map;     if (event.currentTarget.selectedValue == "on"){          map.getLayer(RBlabel).visible = true;       RBonsel = true;                                                 RBoffsel = false;      }     if (event.currentTarget.selectedValue == "off"){         map.getLayer(RBlabel).visible = false;         RBoffsel = true;                           RBonsel = false;           }    }    private function infoWinClose_Handler(event:Event):void    {            var map:Map = hostComponent.map;      map.getLayer(RBlabel).visible = false;      RBoffsel = true;      RBonsel = false;     }     }    //End My add   ]]>  </fx:Script>   <fx:Declarations>   <!--- @private -->   <s:RadioButtonGroup id="layerRBgroup" change="layerRBgroup_changeHandler(event)" />     <!--- @private -->   <s:HGroup gap="6" verticalAlign="middle" id="ToggleLayers">    <s:RadioButton id="on" value="on" group="{layerRBgroup}" label="Turn Labels On" selected="{RBonsel}" />    <s:RadioButton id="off" value="off" group="{layerRBgroup}" label="Turn Labels Off" selected="{RBoffsel}" />   </s:HGroup>  </fx:Declarations> </s:SparkSkin>Solved! Go to Solution.
    attachmentInspector.addEventListener(AttachmentMouseEvent.ATTACHMENT_CLICK, attachmentInspector_attachmentClickHandler);
    hostComponent.parent.parent.parent.parent.addEventListener("close", infoWinClose_Handler);
   }
   
   private function attachmentInspector_attachmentClickHandler(event:AttachmentMouseEvent):void            private function infoWinClose_Handler(event:Event):void
            {
                hostComponent.parent.parent.parent.parent.removeEventListener("close", infoWinClose_Handler)
                var map:Map = hostComponent.map;
                var lyr:Layer = map.getLayer(RBlabel);
                if(lyr){
                    lyr.visible = false;
                }
                RBoffsel = true;
                RBonsel = false;
            }
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		            private function skin_preinitializeHandler(event:FlexEvent):void
            {
                // set the link styles
     if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/Buildings2/MapServer/0"){
   
     RBlayer = "Buildings";
     RBlabel = "Building Labels";
    }TypeError: Error #1009: Cannot access a property or method of a null object reference. at com.esri.ags.skins::PopUpRendererSkin/skin_preinitializeHandler()at com.esri.ags.skins::PopUpRendererSkin/___PopUpRendererSkin_SparkSkin1_preinitialize() at flash.events::EventDispatcher/dispatchEventFunction() at flash.events::EventDispatcher/dispatchEvent() at mx.core::UIComponent/dispatchEvent() at mx.core::UIComponent/initialize() at com.esri.ags.skins::PopUpRendererSkin/initialize() at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded() at mx.core::UIComponent/addChild() at spark.components.supportClasses::SkinnableComponent/attachSkin() at spark.components.supportClasses::SkinnableComponent/validateSkinChange() at spark.components.supportClasses::SkinnableComponent/createChildren() at mx.core::UIComponent/initialize() at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded() at mx.core::UIComponent/addChildAt() at spark.components::Group/addDisplayObjectToDisplayList() at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded() at spark.components::Group/addElementAt() at spark.components::Group/addElement() at com.esri.ags.components::ContentNavigator/commitProperties() at mx.core::UIComponent/validateProperties() at mx.managers::LayoutManager/validateProperties() at mx.managers::LayoutManager/doPhasedInstantiation() at mx.managers::LayoutManager/doPhasedInstantiationCallback() 
Rhett,
It is simple the reason you are getting a null is that the hostComponent.featureLayer is null and this evaluate the url of the featurelayer if the featurelayer is null.
   private function skin_preinitializeHandler(event:FlexEvent):void
   {
    // set the link styles
         if (hostComponent.featureLayer.url != null){
             if (hostComponent.featureLayer.url == "http://gis01.wch-rcc.com/ArcGIS/rest/services/Base/Buildings2/MapServer/0"){
   
     RBlayer = "Buildings";
     RBlabel = "Building Labels";
    }
}if(hostComponent.featureLayer){
//do something
}
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		