To reproduce:

public Form1()
{
    InitializeComponent(); 
    List<Item> items = new List<Item>();
    items.Add(new Item(1,"sample"));
    items.Add(new Item(2, null));
    items.Add(new Item(3, "sample2"));
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.EnableFiltering = true;
    this.radGridView1.ShowFilteringRow = false;
    this.radGridView1.ShowHeaderCellButtons = true; 
}

public class Item
{
    public int Id { get; set; }

    public string Description { get; set; }

    public Item(int id, string description)
    {
        this.Id = id;
        this.Description = description;
    }
}

Workaround: If possible, instead of using null value, use empty string.
If not possible, you will have to employ 3 classes - MyFilterMenuTreeElement, MyFilterMenuTreeItem, MyListFilterPopup. The classes are provided in the attached project RadGridViewFiltering.zip. Once you add the classes to your project, all you have to do is to replace the default popup with the new one, in the FilterPopupRequired handler: 
		private void RadGridView1_FilterPopupRequired(object sender, FilterPopupRequiredEventArgs e)
        {
            if (e.FilterPopup is RadListFilterPopup)
            {
                e.FilterPopup = new MyListFilterPopup(e.Column);
            }
        }
		
The approach is also demonstrated in the attached project.