Hi, 

Thanks for the sample!

So the reason for the exception is your code right here:

 private void XamGrid_CellExitedEditMode(object sender, CellExitedEditingEventArgs e)

 {

            this.XamGrid.InvalidateData();

}

Basically, calling InvalidateData causes all data operations to be performed again, which means all GroupBy info will be undone, and a new GroupBy operation will be done.  And, considering where you're doing it, its causing other operations to try and perform while they're in an invalid state. 

So, what exactly are you trying to accomplish?  Note that InvalidateData is an expensive operation, and should only be done when absolutely necessary.  Also, if your data implements INotifyPropertyChanged, there would be no need for it at all. 

-SteveZ