Adds a null check to ensure the current application is not null when we try to update the list of active applications

Author: AKruimink

src/WinReform/WinReform/ActiveWindows/ActiveWindowsViewModel.cs

@@ -157,7 +157,7 @@ public void RefreshActiveWindows()
             Task.Run(() =>
             {
                 var result = _windowService.GetActiveWindows().ToList();
-                Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() => ActiveWindows.UpdateCollection(result)));
+                Application.Current?.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() => ActiveWindows.UpdateCollection(result)));
             });
         }