validateExternalPluginsType(plugins) {
      const pluginTypes = ['playback', 'container', 'core']
      pluginTypes.forEach((type) => {
        (plugins[type] || []).forEach((el) => {
          const errorMessage = 'external ' + el.type + ' plugin on ' + type + ' array'
          if (el.type !== type) throw new ReferenceError(errorMessage)
        })
      })
    }
validate if the external plugins that were passed through `options.plugins` are associated to the correct type
@method validateExternalPluginsType
@private
@param {Object} plugins the config object with all plugins