old_contents: from cms.plugin_base import CMSPluginBase
  
  
  class SPAPluginMixin(object):
      frontend_component_name = None
  
      def render_spa(self, request, context, instance):
          return context
  
  
  class SPAPluginBase(SPAPluginMixin, CMSPluginBase):
      render_template = 'djangocms_spa/empty.html'  # The plugin needs a template although we render JSON only.
  
new_contents: from cms.plugin_base import CMSPluginBase
  
  
  class SPAPluginMixin(object):
      frontend_component_name = None
      parse_child_plugins = True
  
      def render_spa(self, request, context, instance):
          return context
  
  
  class SPAPluginBase(SPAPluginMixin, CMSPluginBase):
      render_template = 'djangocms_spa/empty.html'  # The plugin needs a template although we render JSON only.
  
subject: Add default value