Problem and motivation: We are trying to recreate https://www.drupal.org/project/menu_editor for Drupal 8. So our idea is to extend MenuForm and then override the form in our own way to achieve what menu_editor for Drupal 7 provides.

What we have done so far

We created a custom module called menu_editor and there we created a route

  entity.menu.power_edit_form:
  path: '/admin/structure/menu/manage/power_edit/{menu}'
  defaults:
    _form: '\Drupal\menu_editor\Form\MenuEditor'
    _title_callback: '\Drupal\menu_ui\Controller\MenuController::menuTitle'
  requirements:
    _permission: 'access content'

No inside our MenuEditor form we extend MenuForm

<?php

namespace Drupal\menu_editor\Form;
 

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Menu\MenuLinkManagerInterface;
use Drupal\Core\Menu\MenuLinkTreeInterface;
use Drupal\Core\Utility\LinkGeneratorInterface;
use Drupal\menu_link_content\MenuLinkContentStorageInterface;
use Drupal\menu_ui\MenuForm;
use Drupal\system\MenuInterface;

/**
 * Class MenuEditor.
 */
class MenuEditor extends MenuForm {

  var $menu ;

 public function __construct(MenuLinkManagerInterface $menu_link_manager, MenuLinkTreeInterface $menu_tree, LinkGeneratorInterface $link_generator, MenuLinkContentStorageInterface $menu_link_content_storage)
 {
   // Trying to inject menu as suggested in https://drupal.stackexchange.com/questions/219431/entity-form-additional-arguments but not sure how we can get menu object over here
   $form_object = \Drupal::entityTypeManager()->getFormObject('menu', 'edit');
   $form_object->setEntity($this->menu);
   parent::__construct($menu_link_manager, $menu_tree, $link_generator, $menu_link_content_storage);
 }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'menu_editor';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, MenuInterface $menu = NULL) {
   // Can get menu object over here but not sure how I can pass this in constructor. 
   $this->menu = $menu;
    
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    parent::submitForm($form, $form_state);
  }
 

}

Now whenever we are running this code, we get the following error.

TypeError: Argument 1 passed to Drupal\Core\Entity\EntityForm::setEntity() must implement interface Drupal\Core\Entity\EntityInterface, null given,

I do understand that using $this in the form will not help, since the constructor is called before, but not sure how to accomplish this.

We are trying to recreate https://www.drupal.org/project/menu_editor for Drupal 8. So our idea is to extend MenuForm and then override the form in our own way to achieve what menu_editor for Drupal 7 provides.

We created a custom module called menu_editor and there we created a route.

  entity.menu.power_edit_form:
  path: '/admin/structure/menu/manage/power_edit/{menu}'
  defaults:
    _form: '\Drupal\menu_editor\Form\MenuEditor'
    _title_callback: '\Drupal\menu_ui\Controller\MenuController::menuTitle'
  requirements:
    _permission: 'access content'

Inside MenuEditor, we extend MenuForm.

namespace Drupal\menu_editor\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Menu\MenuLinkManagerInterface;
use Drupal\Core\Menu\MenuLinkTreeInterface;
use Drupal\Core\Utility\LinkGeneratorInterface;
use Drupal\menu_link_content\MenuLinkContentStorageInterface;
use Drupal\menu_ui\MenuForm;
use Drupal\system\MenuInterface;

/**
 * Class MenuEditor.
 */
class MenuEditor extends MenuForm {

  var $menu;

  public function __construct(MenuLinkManagerInterface $menu_link_manager, MenuLinkTreeInterface $menu_tree, LinkGeneratorInterface $link_generator, MenuLinkContentStorageInterface $menu_link_content_storage)
 {
   // Trying to inject menu as suggested in https://drupal.stackexchange.com/questions/219431/entity-form-additional-arguments but not sure how we can get menu object over here
   $form_object = \Drupal::entityTypeManager()->getFormObject('menu', 'edit');
   $form_object->setEntity($this->menu);
   parent::__construct($menu_link_manager, $menu_tree, $link_generator, $menu_link_content_storage);
 }

 /**
  * {@inheritdoc}
  */
 public function getFormId() {
   return 'menu_editor';
 }

 /**
  * {@inheritdoc}
  */
 public function buildForm(array $form, FormStateInterface $form_state, MenuInterface $menu = NULL) {
   // Can get menu object over here but not sure how I can pass this in constructor. 
   $this->menu = $menu; 
 }

 /**
  * {@inheritdoc}
  */
 public function submitForm(array &$form, FormStateInterface $form_state) {

   parent::submitForm($form, $form_state);
 }

}

Whenever we are running this code, we get the following error.

TypeError: Argument 1 passed to Drupal\Core\Entity\EntityForm::setEntity() must implement interface Drupal\Core\Entity\EntityInterface, null given

I do understand that using $this in the form will not help, since the constructor is called before, but I am not sure how to accomplish this.