Still getting exception NewItemPlaceholderPosition' is not allowed during a transaction begun by 'AddNew'

Hi, i'm trying to create my first MVVM application so i have:

1. ViewModel containing ObserveraleCollection TeamsCollection, where items of TeamsCollection are type of Team. Each Team contains collection of type EntitySet<Player> Players and property "TeamIsNotFull" which returns true if count of players is below 2 othervise returns false. 
2. VievModel is connected to  View whitch contains two datagrids. The Datagrid1 shows all teams in the TeamsCollection and one cell of each row contains inner Datagrid2 which shows Players of team. Property "TeamIsNotFull" is binded to property CanUserAddRows of the Datagrid2. Everything works well. But at the moment when I delete a row with some player and add  a new row i'm getting exception  NewItemPlaceholderPosition' is not allowed during a transaction begun by 'AddNew'.
3. Sorry for my bad english. :-)