Avoid "No widget" errro messages

src/lib/y2firewall/widgets/services_table.rb

@@ -71,13 +71,20 @@ def items
       def services=(services)
         old_services = @services
         @services = services
-        change_items(items)
+        change_items(items) if widget_exists?
 
         return if Yast::UI.TextMode
 
         new_services = services - old_services
         self.value = new_services
       end
+
+      # Determines whether the widget exists in the UI
+      #
+      # @return [Boolean]
+      def widget_exists?
+        Yast::UI.WidgetExists(Id(widget_id))
+      end
     end
   end
 end