ObservableCollection throw exception when load second time

Question

I have an ObservableCollection that when i load data into it one time and when i load again it thorws an exception the object is not set on reference

using (PricingManagerEntities db = new PricingManagerEntities())
                    {
                        m_PricingSheetManagerViewModel.Groups = new ObservableCollection<sp_GetGroups_Result>(db.sp_GetGroups(null, true));
                    }
                    return m_PricingSheetManagerViewModel;

I did the follwing:

1. spilt the line into servel lines:

   var a = db.sp_GetGroups(null, true).ToList();
   b = new ObservableCollection<sp_GetGroups_Result>(a);
   m_PricingSheetManagerViewModel.Groups = b;
   

   and notice that it throws when: m_PricingSheetManagerViewModel.Groups = b;

I tried to clear the Groups try to remove the items, but i always getting System.NullReferenceException

the first time I initialize the Groups the Groups is null, and getting 2 records then i select one of the groups via combobox and exit the window, next i enter again and it throws me.

here's the combobox xaml:

<ComboBox HorizontalAlignment="Left" Width="200"  Name="cmbGroups" ItemsSource="{Binding Groups}" Height="30" DisplayMemberPath="GroupName" SelectionChanged="cmbGroup_changed" HorizontalContentAlignment="Center" Margin="5"/>

here's when selection changed:

private void cmbGroup_changed(object sender, SelectionChangedEventArgs e)
    {
        long groupID = ((sender as ComboBox).SelectedValue as sp_GetGroups_Result).GroupID;
        m_engine.GetItemsByGroupID(groupID);
    }
internal void GetItemsByGroupID(long groupID)
    {
        if (m_PricingSheetManagerViewModel.Items == null)
            m_PricingSheetManagerViewModel.Items = new ObservableCollection<sp_GetItems_Result>();

        m_PricingSheetManagerViewModel.Items.Clear();
        using (PricingManagerEntities db = new PricingManagerEntities())
        {
            List<ItemsInGroups> groups = db.ItemsInGroups.Where(iig => iig.GroupID == groupID).ToList();
            foreach(ItemsInGroups group in groups)
            {
                var temp = db.sp_GetItems(group.ItemID, true).FirstOrDefault();
                if(temp != null)
                    m_PricingSheetManagerViewModel.Items.Add(temp);
            }
        }
    }