Ensures picker selection updates on UI thread

* Addresses potential threading issues by ensuring picker selection changes and event invocations occur on the UI thread.
* Enhances robustness by adding null checks for the associated UI element.
* Refines the update logic to prevent unnecessary or redundant assignments to the UI element's selected item.

Author: michaelstonis

Stellar.Maui/Extensions/PickerExtensions.cs

@@ -437,13 +437,21 @@ private void SetSelectedItem(bool fromNotificationTrigger = false)
 
     private void SelectedItemChanged(TViewModel? item, bool fromUi = false)
     {
-        _selectedItemChanged?.Invoke(item);
+        var picker = _picker;
+
+        if (picker is null)
+        {
+            return;
+        }
+
+        picker.Dispatcher?.Dispatch(() => _selectedItemChanged?.Invoke(item));
 
-        if (!fromUi && _picker is not null && (_picker.SelectedItem is null || !EqualityComparer<TViewModel>.Default.Equals(item, this.SelectedItem)))
+        if (fromUi || (_picker.SelectedItem is not null && EqualityComparer<TViewModel>.Default.Equals(item, this.SelectedItem)))
         {
-            var picker = _picker;
-            picker?.Dispatcher?.Dispatch(() => picker.SelectedItem = item);
+            return;
         }
+
+        picker.Dispatcher?.Dispatch(() => picker.SelectedItem = item);
     }
 
     public void Dispose()