Editing menu results in 500 error

Problem/Motivation

Clicking edit icon on menu item in a page's (contrib) tb_megamenu menu block results in progress arrow continuously and browser console show AJAX call had returned HTTP 500. Expanding this shows exception text: 'Object of type ...Entity\Menu cannot be printed.'

Steps to reproduce

Clicking edit icon on menu item in a page's menu block.

To reproduce standalone:
Set up hook_theme() in a test .module and twig template as described in https://www.drupal.org/docs/develop/theming-drupal/twig-in-drupal/create...

Add to test module Controller:

use Drupal\system\Entity\Menu;
....
$m = new Menu(['id'=>'mymenuid', 'label'=>'mymenulabel'], 'menu');
return [
      '#theme' => 'my_template',
      '#test_var' => $m,
    ];

Calling controller will give the:

Exception: Object of type Drupal\system\Entity\Menu cannot be printed. in Drupal\Core\Template\TwigExtension->escapeFilter() (line 459 of core/lib/Drupal/Core/Template/TwigExtension.php). 
__TwigTemplate_570b34ddc31fd2054d7b637ea3a5d243->doDisplay() (Line: 388)
....

Proposed resolution

The Exception is thrown in core/lib/Drupal/Core/Template/TwigExtension.php. This filter looks for a toString() etc method on the Menu entity (in core/modules/system/src/Entity/Menu.php)
Resolve by adding a simple toString() method to Menu entity, returning the label or id property (see patch).
Rerun test above, and you should see

Test twig template!
 
test_var: mymenulabel

Remaining tasks

Apply patch to current and future branches

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet