Flex :: Composite MXML Component - Display File Info On Single Line

I'm trying componentize one of the pieces of UI in an AIR application that I'm developing in Flex. In this example, I want to display file information on a single line (which has an icon, some text/link and the size).

My code looks like this (component is called FileDisplay):
<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns:mx="[URL]"><mx:Script>
<![CDATA[
public function set iconType(source:String):void {
this.ficon.source = source;
[Code] .....

However, when I do this, I get an error: Error #1009: Cannot access a property or method of a null object reference. This is because the child components of the FileDisplay are null (or at least they show up that way in the debugger). Am I supposed to be waiting for events indicating the child components were created? For now I can manually do everything in ActionScript in my main app (create a Canvas and add children to it) but how to separate the code more cleanly.