Rickey,
The filterLayer = lLayer; set the local var of filterLayer to the current layer the widget is configuring and this var is used by the applyFilter function to set the definition for that layer based on the current filter settings. I would get the layer from the map based on the selected tab. Something like this._viewerMap.getLayer(the layers name); maybe in a function like this:
getCurrentSelectedFilterLayer: function(layerName){
return this._viewerMap.getLayer(layerName);
},
That way you can call it from the applyFilter function. Or even better use the function to set the filterLayer to the proper layer when the tab is changed.