Checkbox field gets a PHP E_NOTICE for a missing 'multiple' index in the $field array.

Hey @elliotcondon

Recently I've updated the ACF PRO from 5.8.9 to 5.8.11 and started to get the E_NOTICE when I'm trying to create a checkbox.

The $field is missing a multiple index

Specs:

• PHP: 7.2
• WP: 5.4.1
• ACF: 5.8.11

The stack trace of the error is the following:

ErrorException thrown with message "Undefined index: multiple"

Stacktrace:
#17 ErrorException in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/fields/class-acf-field-select.php:492
#16 Themosis\Core\Bootstrap\ExceptionHandler:handleError in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/fields/class-acf-field-select.php:492
#15 acf_field_select:update_field in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/fields/class-acf-field-checkbox.php:445
#14 acf_field_checkbox:update_field in /pathToProject/htdocs/cms/wp-includes/class-wp-hook.php:287
#13 WP_Hook:apply_filters in /pathToProject/htdocs/cms/wp-includes/plugin.php:249
#12 apply_filters_ref_array in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/acf-hook-functions.php:101
#11 _acf_apply_hook_variations in /pathToProject/htdocs/cms/wp-includes/class-wp-hook.php:287
#10 WP_Hook:apply_filters in /pathToProject/htdocs/cms/wp-includes/plugin.php:206
#9 apply_filters in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/acf-field-functions.php:971
#8 acf_update_field in /pathToProject/htdocs/content/mu-plugins/advanced-custom-fields-pro/includes/admin/admin-field-group.php:488
#7 acf_admin_field_group:save_post in /pathToProject/htdocs/cms/wp-includes/class-wp-hook.php:289
#6 WP_Hook:apply_filters in /pathToProject/htdocs/cms/wp-includes/class-wp-hook.php:311
#5 WP_Hook:do_action in /pathToProject/htdocs/cms/wp-includes/plugin.php:478
#4 do_action in /pathToProject/htdocs/cms/wp-includes/post.php:4153
#3 wp_insert_post in /pathToProject/htdocs/cms/wp-includes/post.php:4244
#2 wp_update_post in /pathToProject/htdocs/cms/wp-admin/includes/post.php:409
#1 edit_post in /pathToProject/htdocs/cms/wp-admin/post.php:227
#0 require in ~/.composer/vendor/laravel/valet/server.php:158

The Content of that $field:

array:1 [▼
  "field" => array:24 [▼
    "ID" => 13026
    "key" => "field_5ecb95ae4f017"
    "label" => "Checkbox Test"
    "name" => "testing"
    "prefix" => ""
    "type" => "checkbox"
    "value" => null
    "menu_order" => 0
    "instructions" => ""
    "required" => 0
    "id" => ""
    "class" => ""
    "conditional_logic" => 0
    "parent" => 13025
    "wrapper" => array:3 [▶]
    "choices" => array:2 [▼
      1 => "CheckBox"
      2 => "Select"
    ]
    "allow_custom" => 0
    "default_value" => []
    "layout" => "vertical"
    "toggle" => 0
    "return_format" => "value"
    "_name" => "testing"
    "_valid" => 1
    "save_custom" => 0
  ]
]

Start at includes/fields/class-acf-field-select.php:492 and follow the update_field call from includes/fields/class-acf-field-checkbox.php:445. Reproduce saving the checkbox field with the shown $field data, then verify the missing multiple index no longer produces an E_NOTICE.