Because you can put nulls in your list, even though you probably won't
you can do
foreach (UIElement uiElement in list.Where(e => e != null))
{
uiElement.SetValue(Grid.ColumnProperty, colunmn++);
uiElement.SetValue(Grid.RowProperty, _uiRoot.RowDefinitions.Count -1);
_uiRoot.Children.Add(uiElement);
}