Fatal error while editing and deleting menu items
This error was coming because of responsive menu module.Responsive menu was fetching the parent links using 

\Drupal::service('plugin.manager.menu.link')-&gt;createInstance($parent);


Adding the condition before calling createInstance will fix this issue.

\Drupal::service('plugin.manager.menu.link')-&gt;hasDefinition($parent)


Created a issue on D.o also 

https://www.drupal.org/project/1786786/issues/2924229