Custom itemRenderer component based on cell value: error 1009

Question

I'm working on an item renderer for a dataGrid that has different states depending on the cell and row values.
The cell value is a toggle (true or null), and sets whether content should be shown in the cell or not
The row properties determine what is shown when the cell value is true.
The dataGrid dataProvider is populated based on user id input.
I created the itemRenderer as a custom actionscript component, closely following this example:
360Flex Sample: Implementing IDropInListItemRenderer to create a reusable itemRenderer
However, my component results in Error #1009 (Cannot access a property or method of a null object reference) when a user id is submitted.
package components
     import mx.containers.VBox;
     import mx.controls.*;     import mx.controls.dataGridClasses.DataGridListData;
     import mx.controls.listClasses.BaseListData;
     import mx.core.*;
     public class toggleCellRenderer extends VBox
          public function ToggleCellRenderer()
          {super();}
          private var _listData:BaseListData;   
               private var cellState:String;
               private var cellIcon:Image;
               private var imagePath:String;
               private var imageHeight:int;
               private var qty:String = data.qtyPerTime;
               private var typ:String = data.type;
          public function get listData():BaseListData
               {return _listData;}
          public function set listData(value:BaseListData):void
               {_listData = value;}
          override public function set data(value:Object):void {
               super.data = value;
               if (value != null)
               //errors on next line: Error #1009: Cannot access a property or method of a null object reference.
               {cellState = value[DataGridListData(_listData).dataField]}
          override protected function createChildren():void {
               removeAllChildren();
               if(cellState=='true'){
                    cellIcon = new Image();
                    addChild(cellIcon);
               //there is another state here that adds another child...
          //next overrides commitProperties()...
There are no errors if I don't use an itemRenderer--the cells correctly toggle between "true" and empty when clicked.
I also tried a simple itemRenderer component that disregards the cell value and shows in image based off row data--this works fine without errors or crashing. But I need to tie it to the cell value!
I have very limited experience programming, in Flex or any other language. Any help would be appreciated.

Answer 1

Your assumption that the xml file either loads with "true" or nothing  is right.
After modifying the code to the following, I don't get the error, but it's still not reading the cell value correctly.
package components
     import mx.containers.VBox;
     import mx.controls.*;   
     import mx.controls.dataGridClasses.DataGridListData;
     import mx.controls.listClasses.BaseListData;
     import mx.core.*;
     public class toggleCellRenderer extends VBox
          public function ToggleCellRenderer()
           super();
          private var _listData:BaseListData;   
          private var cellState:Boolean;
          private var cellIcon:Image;
          private var imagePath:String;
          private var imageHeight:int;
          private var qty:String;
          private var typ:String;
           public function get listData():BaseListData
             return _listData;
          override public function set data(value:Object):void {
               cellState = false;
               if (listData && listData is DataGridListData && DataGridListData(listData).dataField != null){
                   super.data = value;
                   if (value[DataGridListData(this.listData).dataField] == "true"){
                       cellState = true;
          override protected function createChildren():void {
               removeAllChildren();
               if(cellState==true){
                    cellIcon = new Image();
                    addChild(cellIcon);
               //there is another state here that adds another child...
          //next overrides commitProperties()...
- didn't set the value of qty or typ in the variable declarations (error 1009 by this too--I removed this before but wanted to point out in case its useful)
- added back in the get listData() function so I could use the listData
- changed the null check
All cells are still returning cellState = false when some are set to true, even if I comment out [if (value[DataGridListData(this.listData).dataField] == "true")] and just have it look for non-null data. That shouldn't make a difference anyway, but it confirms that all cells are returning null value.
Swapping out the first if statement in set data with different variables results in the following:
[if (listData != null)]  all cells return null (cellState=false for all)
both [if (value != null)] and  [if (DataGridListData != null)]  results in error 1009 on a line following the if, so I assume they return non-null values.
All rows have data, just not all fields in all rows, so shouldn't listData be non-null?  Could it be that the xml file hasn't fully loaded before the itemRenderer kicks in?
I also realized  I had removed the item renderer from many of the columns for testing, and since some columns are hidden by default only one column in the view was using the itemRenderer--hence the single alert per row I was worried about earlier.
Thanks for your help so far.