Try to fix problem with ComboBox.

Only the OS can create the root automation peer, so we need to take care to not accidentally try to create it from `PopupAutomationPeer`.

src/Avalonia.Controls/Automation/Peers/PopupAutomationPeer.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using Avalonia.Automation.Platform;
 using Avalonia.Controls;
+using Avalonia.Controls.Diagnostics;
 using Avalonia.Controls.Primitives;
 using Avalonia.VisualTree;
 
@@ -20,9 +21,9 @@ namespace Avalonia.Automation.Peers
 
         protected override IReadOnlyList<AutomationPeer>? GetChildrenCore()
         {
-            var host = (IVisualTreeHost)Owner;
-            System.Diagnostics.Debug.WriteLine($"Popup children='{host}'");
-            return host.Root is Control c ? new[] { GetOrCreatePeer(c) } : null;
+            var popupHost = ((IPopupHostProvider)Owner)?.PopupHost as Control;
+            var hostPeer = popupHost?.GetAutomationPeer();
+            return hostPeer is object ? new[] { hostPeer } : null;
         }
 
         protected override bool IsContentElementCore() => false;
@@ -45,8 +46,8 @@ namespace Avalonia.Automation.Peers
 
         private AutomationPeer? GetPopupRoot()
         {
-            var popupRoot = ((IVisualTreeHost)Owner).Root as Control;
-            return popupRoot is object ? GetOrCreatePeer(popupRoot) : null;
+            var popupRoot = ((IPopupHostProvider)Owner).PopupHost as Control;
+            return popupRoot?.GetAutomationPeer();
         }
     }
 }

src/Avalonia.Controls/Control.cs

@@ -235,6 +235,12 @@ namespace Avalonia.Controls
             return new NoneAutomationPeer(factory, this);
         }
 
+        internal AutomationPeer? GetAutomationPeer()
+        {
+            VerifyAccess();
+            return _automationPeer;
+        }
+
         internal AutomationPeer GetOrCreateAutomationPeer(IAutomationNodeFactory factory)
         {
             VerifyAccess();