Dismiss the details pane when the list gets emptied (#39206)

I cannot find an issue for this. I swear I filed it somewhere.

If you open winget, search for "terminal", wait till it loads, then
hit `esc`, we'll clear the search and empty the list, but never actually
hide the details pane. That looks weird.

This fixes that.

Closes _nothing i guess_.

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListViewModel.cs

@@ -328,7 +328,19 @@ public partial class ListViewModel : PageViewModel, IDisposable
     }
 
     [RelayCommand]
-    private void UpdateSelectedItem(ListItemViewModel item)
+    private void UpdateSelectedItem(ListItemViewModel? item)
+    {
+        if (item != null)
+        {
+            SetSelectedItem(item);
+        }
+        else
+        {
+            ClearSelectedItem();
+        }
+    }
+
+    private void SetSelectedItem(ListItemViewModel item)
     {
         if (!item.SafeSlowInit())
         {
@@ -357,6 +369,23 @@ public partial class ListViewModel : PageViewModel, IDisposable
            });
     }
 
+    private void ClearSelectedItem()
+    {
+        // GH #322:
+        // For inexplicable reasons, if you try updating the command bar and
+        // the details on the same UI thread tick as updating the list, we'll
+        // explode
+        DoOnUiThread(
+           () =>
+           {
+               WeakReferenceMessenger.Default.Send<UpdateCommandBarMessage>(new(null));
+
+               WeakReferenceMessenger.Default.Send<HideDetailsMessage>();
+
+               TextToSuggest = string.Empty;
+           });
+    }
+
     public override void InitializeProperties()
     {
         base.InitializeProperties();