add a failing unit test.

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -1091,6 +1091,46 @@ namespace Avalonia.Controls.UnitTests
                 Assert.True(called);
             }
         }
+        
+        [Fact]
+        public void SelectedItem_Should_Be_Valid_When_SelectedItemChanged_Event_Raised()
+        {
+            using (Application())
+            {
+                var tree = CreateTestTreeData();
+                var target = new TreeView
+                {
+                    Template = CreateTreeViewTemplate(),
+                    ItemsSource = tree,
+                };
+
+                var visualRoot = new TestRoot();
+                visualRoot.Child = target;
+
+                CreateNodeDataTemplate(target);
+                ApplyTemplates(target);
+                ExpandAll(target);
+
+                var item = tree[0].Children[1].Children[0];
+                var container = (TreeViewItem)target.TreeContainerFromItem(item);
+
+                Assert.NotNull(container);
+                
+                var called = false;
+                target.SelectionChanged += (s, e) =>
+                {
+                    Assert.Same(item, e.AddedItems[0]);
+                    Assert.Same(item, target.SelectedItem);
+                    called = true;
+                };
+
+                _mouse.Click(container);
+
+                Assert.Equal(item, target.SelectedItem);
+                Assert.True(container.IsSelected);
+                Assert.True(called);
+            }
+        }
 
         [Fact]
         public void Bound_SelectedItem_Should_Not_Be_Cleared_when_Changing_Selection()