FieldReference::buildFieldFormRow() checks the value of $third_party_settings_form, which is never initialized, for example, in the following code.
if ($settings_form || $third_party_settings_form) {
$row['plugin']['#cell_attributes'] = ['colspan' => 3];
$row['plugin']['settings_edit_form'] = [
'#type' => 'container',
'#attributes' => ['class' => ['field-plugin-settings-edit-form']],
'#parents' => ['fields', "display_fields_$field_name", 'settings_edit_form'],
'label' => ['#markup' => t('Plugin settings')],
'settings' => $settings_form,
// 'third_party_settings' => $third_party_settings_form,
'actions' => [
'#type' => 'actions',
'save_settings' => $base_button + [
'#type' => 'submit',
'#button_type' => 'primary',
'#name' => "display_fields_{$field_name}_plugin_settings_update",
'#value' => t('Update'),
'#op' => 'update',
],
'cancel_settings' => $base_button + [
'#type' => 'submit',
'#name' => "display_fields_{$field_name}_plugin_settings_cancel",
'#value' => t('Cancel'),
'#op' => 'cancel',
// Do not check errors for the Cancel button, but make sure we
// get the value of the plugin type select.
'#limit_validation_errors' => [['fields', "display_fields_$field_name", 'type']],
],
],
];
$row['#attributes']['class'][] = 'field-plugin-settings-editing';
}
}
Since there is not a class method that could initialize that variable, that variable should not be used.