TypeError: Cannot access offset of type array in isset or empty in FilterPluginBase::convertExposedInput()

Problem/Motivation

In FilterPluginBase::convertExposedInput(), when the filter is a grouped exposed filter (isAGroup()), the selected group value is read from user input:

  elseif (isset($input[$this->options['group_info']['identifier']])) {                                                                                                                        
    $selected_group = $input[$this->options['group_info']['identifier']];                                                                                                                     
  }    

Nothing guarantees that $selected_group is a scalar. If a user (or a malicious client) submits this parameter as an array — e.g. ?group=[] or ?group[]=foo — $selected_group becomes an
array. That array is then used as an array offset further down:

  if ($selected_group != 'All' && empty($this->options['group_info']['group_items'][$selected_group])) {

which throws:

TypeError: Cannot access offset of type array in isset or empty() in Drupal\views\Plugin\views\filter\FilterPluginBase->convertExposedInput()

This code path is invoked from ViewExecutable::_build() on $this->exposed_data (which can come directly from the request's query parameters), so it can be hit before any form validation
(validateExposed()) runs, on any public view with a grouped exposed filter.

Steps to reproduce

1. Create a view with an exposed filter, with the "Grouped filters" (grouped exposed filter) option enabled.
2. Load the view's page passing the group identifier as an array, e.g. ?[]=1 or ?[]=1&[]=2.
3. Observe the fatal TypeError (white screen / 500 error depending on the environment).

Proposed resolution

Check that $selected_group is actually a scalar before using it as an array offset, and return FALSE (no valid selection) otherwise — the same defensive behavior already applied for other
malformed-input cases in this method.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet