@joachim, But not using !empty condition will allow to create $data element array with empty key.
Instead can we add !empty condition for variable $revision_table everywhere in code. For example please see below line of code. can you please confirm?
if ($revision_table) {
$data[$revision_table]['table']['group'] = $this->t('@entity_type revision', ['@entity_type' => $this->entityType->getLabel()]);
$data[$revision_table]['table']['provider'] = $this->entityType->getProvider();
$views_revision_base_table = $revision_table;
if ($revision_data_table) {
$views_revision_base_table = $revision_data_table;
}
$data[$views_revision_base_table]['table']['entity revision'] = TRUE;
$data[$views_revision_base_table]['table']['base'] = [
'field' => $revision_field,
'title' => $this->t('@entity_type revisions', ['@entity_type' => $this->entityType->getLabel()]),
];
// Join the revision table to the base table.
$data[$views_revision_base_table]['table']['join'][$views_base_table] = [
'left_field' => $revision_field,
'field' => $revision_field,
'type' => 'INNER',
];
if ($revision_data_table) {
$data[$revision_data_table]['table']['group'] = $this->t('@entity_type revision', ['@entity_type' => $this->entityType->getLabel()]);
$data[$revision_data_table]['table']['entity revision'] = TRUE;
$data[$revision_table]['table']['join'][$revision_data_table] = [
'left_field' => $revision_field,
'field' => $revision_field,
'type' => 'INNER',
];
}
// Add a filter for showing only the latest revisions of an entity.
$data[$revision_table]['latest_revision'] = [
'title' => $this->t('Is Latest Revision'),
'help' => $this->t('Restrict the view to only revisions that are the latest revision of their entity.'),
'filter' => ['id' => 'latest_revision'],
];
}