Getting Run Time Errors for "PropertySelection Component" for Second example from Tapestry Documentation

[Amol Yadwadkar <ay...@webifyservices.com>]

Hi All,
This is a Question based on the PropertySelection Component.
I am trying the second example given at Tapestry Component List Documentation  for PropertySelection Component.
Second Example has the following Classes:-

A>> PurchagePage 
B>> ItemSelectionModel
C>> Item

I am using these but I am getting the Runtime Error that :-

"Value for parameter 'model' in component ListDisplay/select is null, and a non-null value is required."

I am populating List at ItemSelectionModel's Constructor. 
Can anybody suggest me where I am going wrong!!!!!! I think there is a problem with a Data Population for the List.

Thankx
Best regds,
Amol