Drupal\Core\Config\ImmutableConfigException: Can not save immutable configuration webform.settings

Problem/Motivation

We get the following error after we save at the webform configuration path /admin/structure/webform/config/handlers/mattermost

Drupal\Core\Config\ImmutableConfigException: Can not save immutable configuration webform.settings. Use \Drupal\Core\Config\ConfigFactoryInterface::getEditable() to retrieve a mutable configuration object on Drupal\Core\Config\ImmutableConfig->save()

Steps to reproduce

Drupal Core: 9.3.3
Webform: 6.1.2
1. Go to the webform configuration path /admin/structure/webform/config/handlers/mattermost
2. Click save.

Proposed resolution

Nope

Remaining tasks

Nope

User interface changes

Nope

API changes

Nope

Data model changes

Nope