Disabling event series date fields throws an exception

Problem/Motivation

An event series' Recur Type field or the associated date fields can't be disabled via hook_form_alter() without triggering an exception on save.

For example, I am trying to alter the series edit form to prevent users from changing the recurrence pattern (and thus deleting/recreating the instances) when additional data has been attached to one of the instances. If I alter the form to disable the recur_type field, then edit and save an event series, the EventCreationService throws an exception.

Steps to reproduce

1. In a custom module, implement the following:
   

   use Drupal\Core\Form\FormStateInterface;
   
   function my_module_form_eventseries_form_alter(&$form, FormStateInterface $form_state, $form_id) {
     $entity = $form_state->getFormObject()->getEntity();
     if (!$entity->isNew()) {
       $form['recur_type']['#disabled'] = TRUE;
     }
   }
   

2. Create a new event series.
3. Edit the series. Note that the recurrence type field is now disabled.
4. Click Save.

Expected result: The event series is saved, keeping its current recurrence type field value.

Actual result: EventCreationService::convertFormConfigToArray() throws an exception.

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\Core\Field\FieldTypePluginManager are: comment, datetime, daterange, file_uri, file, image, link, list_float, list_string, list_integer, path, consecutive_recurring_date, daily_recurring_date, monthly_recurring_date, weekly_recurring_date, text_with_summary, text, text_long, integer, uri, entity_reference, uuid, float, email, timestamp, string, language, created, changed, map, string_long, password, decimal, boolean in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).

Drupal\Core\Plugin\DefaultPluginManager->getDefinition(NULL) (Line: 276)
Drupal\recurring_events\EventCreationService->convertFormConfigToArray(Object) (Line: 159)

Proposed resolution

The problem seems to be on line 225 of EventCreationService.php, where it is using $form_state->getUserInput() rather than $form_state->getValues() to access the submitted form values. The difference is that disabled form fields are not available in getUserInput(), whereas getValues() includes the existing values for any disabled fields.

The solution would be to refactor EventCreationService::convertFormConfigToArray() to use getValues().

Remaining tasks

The proposed resolution would also require some additional processing to transform the data returned by getValues() into the same format as the raw values returned by getUserInput(), so that the rest of convertFormConfigToArray() can continue to function as it does currently.

User interface changes

None.

API changes

None.

Data model changes

None.