Handle RetainSelectionOnReset w/ null source.

src/Avalonia.Controls/SelectionNode.cs

@@ -55,11 +55,7 @@ namespace Avalonia.Controls
                     if (_retainSelectionOnReset)
                     {
                         _selectedItems = new List<object?>();
-
-                        foreach (var i in SelectedIndices)
-                        {
-                            _selectedItems.Add(ItemsSourceView!.GetAt(i));
-                        }
+                        PopulateSelectedItemsFromSelectedIndices();
                     }
                     else
                     {
@@ -94,7 +90,7 @@ namespace Avalonia.Controls
 
                     // Setup ItemsSourceView
                     var newDataSource = value as ItemsSourceView;
-                    
+
                     if (value != null && newDataSource == null)
                     {
                         newDataSource = new ItemsSourceView((IEnumerable)value);
@@ -102,6 +98,7 @@ namespace Avalonia.Controls
 
                     ItemsSourceView = newDataSource;
 
+                    PopulateSelectedItemsFromSelectedIndices();
                     HookupCollectionChangedHandler();
                     OnSelectionChanged();
                 }
@@ -453,7 +450,7 @@ namespace Avalonia.Controls
             {
                 _operation?.Selected(selected);
 
-                if (_selectedItems != null)
+                if (_selectedItems != null && ItemsSourceView != null)
                 {
                     for (var i = addRange.Begin; i <= addRange.End; ++i)
                     {
@@ -877,6 +874,19 @@ namespace Avalonia.Controls
             return selectionState;
         }
 
+        private void PopulateSelectedItemsFromSelectedIndices()
+        {
+            if (_selectedItems != null)
+            {
+                _selectedItems.Clear();
+
+                foreach (var i in SelectedIndices)
+                {
+                    _selectedItems.Add(ItemsSourceView!.GetAt(i));
+                }
+            }
+        }
+
         private List<object?> RecreateSelectionFromSelectedItems()
         {
             var removed = new List<object?>();

tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs

@@ -1596,6 +1596,51 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, raised);
         }
 
+        [Fact]
+        public void RetainSelectionOnReset_Handles_Null_Source()
+        {
+            var data = new ResettingList<string> { "foo", "bar", "baz" };
+            var target = new SelectionModel { RetainSelectionOnReset = true };
+            var raised = 0;
+
+            target.SelectionChanged += (s, e) =>
+            {
+                if (raised == 0)
+                {
+                    Assert.Empty(e.DeselectedIndices);
+                    Assert.Empty(e.DeselectedItems);
+                    Assert.Equal(new[] { new IndexPath(1) }, e.SelectedIndices);
+                    Assert.Empty(e.SelectedItems);
+                }
+                else if (raised == 1)
+                {
+                    Assert.Empty(e.DeselectedIndices);
+                    Assert.Empty(e.DeselectedItems);
+                    Assert.Equal(new[] { new IndexPath(1) }, e.SelectedIndices);
+                    Assert.Equal(new[] { "bar" }, e.SelectedItems);
+                }
+                else if (raised == 3)
+                {
+                    Assert.Empty(e.DeselectedIndices);
+                    Assert.Empty(e.DeselectedItems);
+                    Assert.Empty(e.SelectedIndices);
+                    Assert.Empty(e.SelectedItems);
+                }
+
+                ++raised;
+            };
+
+            target.Select(1);
+            Assert.Equal(1, raised);
+
+            target.Source = data;
+            Assert.Equal(2, raised);
+            Assert.Equal(new[] { new IndexPath(1) }, target.SelectedIndices);
+
+            data.Reset(new[] { "qux", "foo", "bar", "baz" });
+            Assert.Equal(3, raised);
+            Assert.Equal(new[] { new IndexPath(2) }, target.SelectedIndices);
+        }
 
         private int GetSubscriberCount(AvaloniaList<object> list)
         {