Icon picker fatal error

Start at includes/fields/class-acf-field-icon_picker.php line 132 and reproduce the failure with the custom tab and icon filters shown in the issue. Use the supplied ACF block configuration and WordPress/PHP versions to confirm the failing render path. Done means the page editor loads the block with the custom icon set without a fatal error.

Describe the bug
When using the icon picker field type in a custom ACF block with a custom icon set I am getting the following error when trying to edit a page where that block is used:

Fatal error: Uncaught TypeError: Cannot access offset of type string on string in /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/fields/class-acf-field-icon_picker.php:132 Stack trace: #0 /var/www/html/wp-includes/class-wp-hook.php(324): acf_field_icon_picker->render_field(Array) #1 /var/www/html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #2 /var/www/html/wp-includes/plugin.php(565): WP_Hook->do_action(Array) #3 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/acf-hook-functions.php(113): do_action_ref_array('acf/render_fiel...', Array) #4 /var/www/html/wp-includes/class-wp-hook.php(324): _acf_apply_hook_variations(Array) #5 /var/www/html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #6 /var/www/html/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #7 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/acf-field-functions.php(807): do_action('acf/render_fiel...', Array) #8 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/acf-field-functions.php(763): acf_render_field(Array) #9 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/acf-field-functions.php(614): acf_render_field_wrap(Array, 'div', 'field') #10 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/pro/blocks.php(613): acf_render_fields(Array, 'block_32caf81ff...', 'div', 'field') #11 /var/www/html/wp-content/plugins/advanced-custom-fields-pro/pro/blocks.php(543): acf_rendered_block(Array, '', true, 1307, Object(WP_Block)) #12 /var/www/html/wp-includes/class-wp-block.php(586): acf_render_block_callback(Array, '', Object(WP_Block)) #13 /var/www/html/wp-includes/blocks.php(2358): WP_Block->render() #14 /var/www/html/wp-includes/blocks.php(2410): render_block(Array) #15 /var/www/html/wp-includes/class-wp-hook.php(324): do_blocks('<!-- wp:acf/her...') #16 /var/www/html/wp-includes/plugin.php(205): WP_Hook->apply_filters('<!-- wp:acf/her...', Array) #17 /var/www/html/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php(1961): apply_filters('the_content', '<!-- wp:acf/her...') #18 /var/www/html/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php(655): WP_REST_Posts_Controller->prepare_item_for_response(Object(WP_Post), Object(WP_REST_Request)) #19 /var/www/html/wp-includes/rest-api/class-wp-rest-server.php(1292): WP_REST_Posts_Controller->get_item(Object(WP_REST_Request)) #20 /var/www/html/wp-includes/rest-api/class-wp-rest-server.php(1125): WP_REST_Server->respond_to_request(Object(WP_REST_Request), '/wp/v2/pages/(?...', Array, NULL) #21 /var/www/html/wp-includes/rest-api.php(586): WP_REST_Server->dispatch(Object(WP_REST_Request)) #22 /var/www/html/wp-includes/rest-api.php(2973): rest_do_request(Object(WP_REST_Request)) #23 [internal function]: rest_preload_api_request(Array, '/wp/v2/pages/13...') #24 /var/www/html/wp-includes/block-editor.php(754): array_reduce(Array, 'rest_preload_ap...', Array) #25 /var/www/html/wp-admin/edit-form-blocks.php(114): block_editor_rest_api_preload(Array, Object(WP_Block_Editor_Context)) #26 /var/www/html/wp-admin/post.php(187): require('/var/www/html/w...') #27 {main} thrown in /var/www/html/wp-content/plugins/advanced-custom-fields-pro/includes/fields/class-acf-field-icon_picker.php on line 132

To Reproduce
Steps to reproduce the behavior:

1. Create an ACF block with an icon picker field with a custom icon set.
2. Use the following code (or similar) in functions.php to manage the custom icon set

add_filter('acf/fields/icon_picker/tabs', function(array $tabs): array {
    $tabs['nwlp'] = 'NWLP';
    return $tabs;
});

add_filter('acf/fields/icon_picker/nwlp/icons', function(array $icons): array {
    $base_url = get_template_directory_uri() . '/assets/images/icons/';

    return [
        [
            'url'   => $base_url . 'accreditations.svg',
            'key'   => 'accreditations',
            'label' => 'Accreditations',
        ],
        [
            'url'   => $base_url . 'audio-podcast.svg',
            'key'   => 'audio-podcast',
            'label' => 'Audio podcast',
        ]
    ];
});

3. Try to insert/edit a page where this block is embedded and the error should show.

Expected behavior
No error to show

Code

{
    "key": "group_687639d742f8f",
    "title": "Block: Icon",
    "fields": [
        {
            "key": "field_687639d70f893",
            "label": "Icon",
            "name": "icon",
            "aria-label": "",
            "type": "icon_picker",
            "instructions": "",
            "required": 0,
            "conditional_logic": 0,
            "wrapper": {
                "width": "",
                "class": "",
                "id": ""
            },
            "tabs": [
                "nwlp"
            ],
            "return_format": "array",
            "allow_in_bindings": 0,
            "library": "all",
            "default_value": {
                "type": null,
                "value": null
            }
        },
        {
            "key": "field_68763a145e4e2",
            "label": "Icon Colour",
            "name": "icon_colour",
            "aria-label": "",
            "type": "select",
            "instructions": "",
            "required": 0,
            "conditional_logic": 0,
            "wrapper": {
                "width": "",
                "class": "",
                "id": ""
            },
            "choices": {
                "blue": "Blue",
                "green": "Green",
                "pink": "Pink"
            },
            "default_value": false,
            "return_format": "value",
            "multiple": 0,
            "allow_null": 0,
            "allow_in_bindings": 0,
            "ui": 0,
            "ajax": 0,
            "placeholder": "",
            "create_options": 0,
            "save_options": 0
        }
    ],
    "location": [
        [
            {
                "param": "block",
                "operator": "==",
                "value": "acf\/icon"
            }
        ]
    ],
    "menu_order": 0,
    "position": "normal",
    "style": "default",
    "label_placement": "top",
    "instruction_placement": "label",
    "hide_on_screen": "",
    "active": true,
    "description": "",
    "show_in_rest": 0,
    "modified": 1752583984
}

Version Information:

• WordPress Version 6.8.2
• PHP Version 8.2.28
• ACF Version ACF Pro 6.4.3
• Browser Chrome 138