ObservableCollection RemoveAt can't remove selecteditem in datagrid

Question

I have two datagrid's in my project and I wanto to move element from first datagrid to second one, and remove it from the first. As far as copying works great i can't remove selected item from first datagrid. Both datargids are binded with observablecollection

 private void Button_Click(object sender, RoutedEventArgs e)
    {
       productionplan.Add(lstorder.SelectedItem as OrderItem); //creates a copy in second datagrid
       orderedlist.RemoveAt(lstorder.SelectedIndex); //should remove from first datagrid
    }

it throws exception "Object reference not set to an instance of an object"
I have no idea why...
Thansk for help!

Answer 1

Instead you can do this,

 private void Button_Click(object sender, RoutedEventArgs e)
    {
       productionplan.Add(lstorder.SelectedItem as OrderItem); 
       orderedlist.Remove(OrderItem);  
    }