Provide support for nested properties with null values. 

Currently when using the following scenario: null ref exception is throw, when manager is null:

private Person _manager;
public Person Manager
{
    get { return _manager; }
    set { UpdateValue(ref _manager, value); }
}

<telerik:DataGridTextColumn PropertyName="Manager.Name" 
                            HeaderText="Manager"/>

Provide support for null values when nested properties are used in datagrid columns.