[flexcoders] Bookmark no longer valid, please help

[luke_lee1124]

Hi, All, 
In my application, 

var tempAC:ArrayCollection = (lastSectionItem.children as
ArrayCollection);
                                                
tempAC.addItemAt(newPage,insertIndex);

//lastSectionItem is the selected node of advanceddatagrid.
//so those codes means add a child to a arraycollection

But i got an odd error: "Bookmark no longer valid". If I ignore this
error, my code works well.

Any ideas?

Thanks