Fix SelectionNode.Cleanup.

- Removed disposed child nodes
- Don't dispose child node with descendent selection

src/Avalonia.Controls/SelectionNode.cs

@@ -342,17 +342,29 @@ namespace Avalonia.Controls
             }
         }
 
-        public void Cleanup()
+        public bool Cleanup()
         {
-            foreach (var child in _childrenNodes)
+            var result = SelectedCount == 0;
+
+            for (var i = 0; i < _childrenNodes.Count; ++i)
             {
-                child?.Cleanup();
+                var child = _childrenNodes[i];
 
-                if (child?.SelectedCount == 0)
+                if (child != null)
                 {
-                    child.Dispose();
+                    if (child.Cleanup())
+                    {
+                        child.Dispose();
+                        _childrenNodes[i] = null;
+                    }
+                    else
+                    {
+                        result = false;
+                    }
                 }
             }
+
+            return result;
         }
 
         public bool Select(int index, bool select)

tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs

@@ -930,6 +930,21 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void Should_Listen_For_Changes_After_Deselect()
+        {
+            var target = new SelectionModel();
+            var data = CreateNestedData(1, 2, 3);
+
+            target.Source = data;
+            target.Select(1, 0);
+            target.Deselect(1, 0);
+            target.Select(1, 0);
+            ((AvaloniaList<object>)data[1]).Insert(0, "foo");
+
+            Assert.Equal(new IndexPath(1, 1), target.SelectedIndex);
+        }
+
         [Fact]
         public void Selecting_Item_Raises_SelectionChanged()
         {