From b96df46054014da28b6ec7f3526b9aa2238882c5 Mon Sep 17 00:00:00 2001
From: Il Harper <afanyiyu@hotmail.com>
Date: Sat, 1 May 2021 23:56:50 +0800
Subject: [PATCH 1/2] fix: Fix ArgumentNullException in
 TreeView.ExpandSubTree()

close #4288
---
 src/Avalonia.Controls/TreeView.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Avalonia.Controls/TreeView.cs b/src/Avalonia.Controls/TreeView.cs
index 3afbbd944c..e890e4346f 100644
--- a/src/Avalonia.Controls/TreeView.cs
+++ b/src/Avalonia.Controls/TreeView.cs
@@ -168,7 +168,7 @@ namespace Avalonia.Controls
 
             var panel = item.Presenter.Panel;
 
-            if (panel != null)
+            if (item.Presenter?.Panel != null)
             {
                 foreach (var child in panel.Children)
                 {

From c6464ba8d55f4713e89de2c2784959c2f740c06f Mon Sep 17 00:00:00 2001
From: Il Harper <afanyiyu@hotmail.com>
Date: Thu, 13 May 2021 21:59:34 +0800
Subject: [PATCH 2/2] fix: Refactor

---
 src/Avalonia.Controls/TreeView.cs | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/Avalonia.Controls/TreeView.cs b/src/Avalonia.Controls/TreeView.cs
index e890e4346f..78cd22ae32 100644
--- a/src/Avalonia.Controls/TreeView.cs
+++ b/src/Avalonia.Controls/TreeView.cs
@@ -166,11 +166,9 @@ namespace Avalonia.Controls
         {
             item.IsExpanded = true;
 
-            var panel = item.Presenter.Panel;
-
             if (item.Presenter?.Panel != null)
             {
-                foreach (var child in panel.Children)
+                foreach (var child in item.Presenter.Panel.Children)
                 {
                     if (child is TreeViewItem treeViewItem)
                     {