This error is thrown when retrieving an item from a collection view
requires an asynchronous call. This error occurs when the backing data
is provided from a remote source and the data is not yet available locally.
If the receiver of this error needs notification when the requested item
becomes available (that is, when the asynchronous call completes), it must
use the addResponder() method and specify
an object that supports the mx.rpc.IResponder
interface to respond when the item is available.
The mx.collections.ItemResponder class implements the
IResponder interface and supports a data property.