def pluginId = plugin.'@id'
            if (!pluginId) {
                throw new RuntimeException("No id specified for plugin: ${plugin.'@description' ?: 'unknown'}")
            }
            plugin.extends.each { Element e ->
                def targetClass = e.'@targetClass'
                if (!targetClass) {
                    throw new RuntimeException("No targetClass specified for extension provided by plugin '$pluginId'.")