(#48) Fix NRE when TaskDialog does not have buttons

Author: augustoproiete

src/Ookii.Dialogs.Wpf/TaskDialog.cs

@@ -1123,7 +1123,7 @@ public TaskDialogButton ShowDialog(IntPtr owner)
             if( IsDialogRunning )
                 throw new InvalidOperationException(Properties.Resources.TaskDialogRunningError);
 
-            if( _buttons.Count == 0 )
+            if (_buttons is null || _buttons.Count == 0)
                 throw new InvalidOperationException(Properties.Resources.TaskDialogNoButtonsError);
 
             _config.hwndParent = owner;