From cdb486fe233ffc5345f58070bcd8b4732c9ba0cd Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 30 Jul 2019 13:39:22 +0200
Subject: [PATCH] Fix NRE in VisualNode.SortChildren.

`_children` may be null. In addition if there are < 2 children, there's no sorting to be done.
---
 src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs  | 5 +++++
 .../Rendering/SceneGraph/VisualNodeTests.cs              | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs b/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
index 709a935450..c3dfe12b2d 100644
--- a/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
+++ b/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
@@ -179,6 +179,11 @@ namespace Avalonia.Rendering.SceneGraph
         /// <param name="scene">The scene that the node is a part of.</param>
         public void SortChildren(Scene scene)
         {
+            if (_children == null || _children.Count <= 1)
+            {
+                return;
+            }
+
             var keys = new List<long>();
 
             for (var i = 0; i < Visual.VisualChildren.Count; ++i)
diff --git a/tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/VisualNodeTests.cs b/tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/VisualNodeTests.cs
index 1101ccacba..24ba2d1c48 100644
--- a/tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/VisualNodeTests.cs
+++ b/tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/VisualNodeTests.cs
@@ -92,5 +92,14 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
             Assert.Same(node1.DrawOperations[0].Item, node2.DrawOperations[0].Item);
             Assert.NotSame(node1.DrawOperations[0], node2.DrawOperations[0]);
         }
+
+        [Fact]
+        public void SortChildren_Does_Not_Throw_On_Null_Children()
+        {
+            var node = new VisualNode(Mock.Of<IVisual>(), null);
+            var scene = new Scene(Mock.Of<IVisual>());
+
+            node.SortChildren(scene);
+        }
     }
 }