Checking _operation for null isn't enough.

Instead check if `UpdateCount` is not 0 to allow recursive setting of `Source`.

I have a vague feeling that this isn't actually the correct solution, and that more issues are going to arise around this scenario but for now I'm unable to create a failing test so let's go with the simple solution for now.

Fixes #11617

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -277,7 +277,7 @@ namespace Avalonia.Controls.Selection
         {
             if (base.Source != value)
             {
-                if (_operation is not null)
+                if (_operation?.UpdateCount > 0)
                 {
                     throw new InvalidOperationException("Cannot change source while update is in progress.");
                 }