Undefined index: full in Drupal\diff\Plugin\diff\Layout\VisualInlineDiffLayout->build()

Problem/Motivation

I am working on developing a diff UI plugin for Taxonomy Terms and came across an issue in which the comparison table for taxonomy term
can not be rendered.

Steps to reproduce

Create a new route in which the path is '/taxonomy/term/{taxonomy_term}/revisions/view/{left_revision}/{right_revision}/{filter}' by a custom module.

In the controller function, use following codes to build the comparison table.

 $build = $this->compareEntityRevisions($route_match, $left_revision, $right_revision, $filter);

When I access following URL:
/taxonomy/term/230/revisions/view/236/237/visual_inline

a fatal PHP error caused said:

Notice: Undefined index: full in Drupal\diff\Plugin\diff\Layout\VisualInlineDiffLayout->build() (line 159 of /usr/local/var/www/govCMS8/docroot/modules/contrib/diff/src/Plugin/diff/Layout/VisualInlineDiffLayout.php)

Proposed resolution

After debugging the codes, I am aware that this issue is caused by line 139 in /src/Plugin/diff/Layout/VisualInlineDiffLayout.php

// Get all view modes for entity type.
    $view_modes = $this->entityDisplayRepository->getViewModeOptionsByBundle($entity->getEntityTypeId(), $entity->bundle());

The problem is that there is no display mode for a taxonomy bundle by default until you go to the display setting page (/admin/structure/taxonomy/manage/{vocabulary name}/overview/display) and save the setting for this vocabulary (bundle).

So $view_modes will be null if you haven't saved the display setting for a vocabulary.

The solution could be using the default display mode for an entity bundle that hasn't had any view display configuration yet. The updated code is

 // Get all view modes for entity type.
    $view_modes = $this->entityDisplayRepository->getViewModeOptionsByBundle($entity->getEntityTypeId(), $entity->bundle());
    if (empty($view_modes)) {
      // There is no any view modes for the entity bundle specified.
      // Use the default view mode of the entity instead.
      $view_modes = $this->entityDisplayRepository->getViewModeOptions($entity->getEntityTypeId());
    }

This could happen to any entity types other than Node, not just Taxonomy Terms.

Remaining tasks

Create test script.

User interface changes

N/A

API changes

N/A

Data model changes

N/A