Settings form crashes if menu_link_content field with lazy enabled exists

Problem/Motivation

The settings form crashes with the following error if a field with lazy enabled exists on the menu_link_content entity type.

Steps to reproduce

1. Install Menu Item Extras and add a lazy loaded image field
2. Visit the settings form of this module

Proposed resolution

Use $entity_type->getBundleEntityType() instead of hardcoding bundle entity types.