This has NOTHING to do with OnAppearing!  The same crash occurs with a ToolbarItem action:

    public class MainPage : ContentPage
    {
        private ObservableCollection<EntityItem> _rows = null;

        public MainPage()
        {
            Title = "Main";
            ToolbarItem loadTBI = new ToolbarItem {
                Text = "Load"
            };
            loadTBI.Clicked += HandleLoadClicked;
            ToolbarItems.Add(loadTBI);
            _rows = new ObservableCollection<EntityItem>();
            RadTreeView treeV = new RadTreeView {
                ItemsSource = _rows
            };
            treeV.Descriptors.Add(new TreeViewDescriptor {
                TargetType = typeof(EntityItem),
                DisplayMemberPath = nameof(EntityItem.Name),
                ItemsSourcePath = nameof(EntityItem.Children)
            });
            Content = treeV;
        }

        private void HandleLoadClicked(object sender, EventArgs e)
        {
            _rows.Clear();
            _rows.Add(new EntityItem { Name = "Hello" });
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();
            //_rows.Clear();
            _rows.Add(new EntityItem { Name = "Hello" });
        }
    }

    public class EntityItem
    {
        public string Name { get; set; }
        public List<EntityItem> Children { get; set; }
    }