fix crash scrolling to view when clearing list.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -1042,7 +1042,7 @@ namespace Avalonia.Controls.Primitives
                 RaiseEvent(e);
             }
 
-            if (AutoScrollToSelectedItem)
+            if (AutoScrollToSelectedItem && _selectedItem != null)
             {
                 ScrollIntoView(_selectedItem);
             }