Got a config schema error on saving a view?

We ran into an obscure error recently, when saving a view that used a custom views plugin. It was supposed to be a very simple extension to core's bundle (content type) filter:

InvalidArgumentException: The configuration property display.default.display_options.filters.bundle.value.article doesn't exist. in Drupal\Core\Config\Schema\ArrayElement->get() (line 76 of [...]/core/lib/Drupal/Core/Config/Schema/ArrayElement.php).

Several contrib projects ran into this issue too: Drupal Commerce, Search API and Webform Views integration. There's even a core issue that looked relevant... but it turned out to be a simple, if perhaps surprising fix. If you ever run into it, it will have a different property (i.e. due to whichever plugin or default value are used).

Our filter was little more than a simple subclass of \Drupal\views\Plugin\views\filter\Bundle, declared for a specific entity type in a very ordinary hook_views_data() (which had even been autogenerated by Drupal Console, so we were fairly confident the problem wasn't there). It just tailored the options form a little to work well for the entity type.

Views plugins all have their own configuration schema - for example, the bundle filter is declared in views.filter.schema.yml to use the 'in_operator', because multiple bundles can be selected for it. When we subclass such a plugin, we do not automatically get to inherit the configuration schema (as that is not part of the PHP class or even its annotation). (Perhaps core could be 'fixed' to recognise this situation ... but there are more important things to work on!)

The solution is to simply copy the schema from the plugin we've extended - in our case, that was 'views.filter.bundle', found in core's 'views.filter.schema.yml' file within views' config/schema sub-directory. Wherever it is, it's probably named 'views.PLUGIN.ID', where 'PLUGIN' is the type of your plugin (e.g. field, filter, area), and 'ID' is the ID in the class annotation of the class your plugin extends. We pasted the schema into our own schema file - which can be named something like /config/schema/mymodule.schema.yml, within our module's directory:

# Replace 'mymodule_special_type' with the ID in your plugin's annotation.
views.filter.mymodule_special_type:
  type: views.filter.in_operator
  label: 'Customised type selector'

Once that file is in place correctly, I expect you just need to rebuild caches and/or the container for Drupal to happy again. Re-save the form, the error is gone :-)

Configuration schemas should normally help development by catching errors, but as I've written before, an incorrect schema can make things surprisingly difficult. I hope someone else finds this article solves their problem so it doesn't take them as long to figure out! I haven't used it, but it's possible that the Configuration inspector project could help you identify issues otherwise.