1. It seems that the handler is broken. It could not be opened and set up via Emails/Handlers page of any webform.
2. 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()
Handler issue.
1. Go to any /admin/structure/webform/manage/contact_form/handlers (webform_id: contact_form)
2. Add handler
3. Select Post to Mattermost
Webform mattermost settings issue
1. Go to the webform configuration path /admin/structure/webform/config/handlers/mattermost
2. Click save.
I provided the patch to fix the error. Please take a look
Fixed
1.0
Code
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Issue #3261458 by cobenash: Mattermost settings and handler couldn't set...
Automatically closed - issue fixed for 2 weeks with no activity.