Changing Image Source programmatically

Question

Hi
I've got a problem changing my image source based on a value in a variable.
The following function gets kicked off, but I get a TypeError: Error #1009: Cannot access a property or method of a null object reference. error.
I've tried embedding, and the solution you see below (which i found here - as I am chaning it at run time http://forums.adobe.com/message/116979#116979), but no luck
I am certain its how I'm mapping the png, but I can't be sure..... any suggestions?  I've been banging my head against my desk for ages on this!
function (note - the trace proves that the value to change the image is correct)
public function LeversComplete():void{
    if(PodContentBase._ProjectDetail.getItemAt(0).IndustryLeadingMastery == "N")
          trace("Value" + PodContentBase._ProjectDetail.getItemAt(0).IndustryLeadingMastery)
            imgIndustry.source = "/assets/bigcheese.png";
     else {imgIndustry.source = "assets/Nocheese.png"} 
Image (note - this is set as a defualt - and defaults in fine)
       <mx:Image id="imgIndustryleadingtechnicalmastery"
                              source="assets/bigcheese.png"/>
Picture of Directory Structure:

Answer 1

Hi - all done - dead right - The image didnt exisit and i'm trying to stuff things in it - what a divvy -
Anyways, this worked -    I check if the image exists before I try and change it.            
public function LeversComplete():void{
          if (imgIndustryleadingtechnicalmastery){
               if(PodContentBase._ProjectDetail.getItemAt(0).IndustryLeadingMastery == "N") {imgIndustryleadingtechnicalmastery.source="assets/nocheese.png"}
               else {imgIndustryleadingtechnicalmastery.source = "assets/bigcheese.png"}