Woocommerce Rest API - How to add new response?

Question

I am trying to insert the $addresscustom to the Woocommerce REST API, but it displays the Error:

> I/flutter (19003): FormatException: Unexpected character (at character 1)
> I/flutter (19003): <style type="text/css"> .wp-die-message {display: none; } 
> I/flutter (19003): ^

Why it is saying the response is not json decoded (Flutter FormatException: Unexpected character (at character 1)), when it is indeed decoded? For instance, when I use the code 2, it works and displays the URL in the REST API:

The code:

function get_address_store( $response, $order_obj, $request, $object) {
    if( empty( $response->data ) )
        return $response;
    
    $order = WC_Order(  $order_obj->get_order_number() );
    $items = $order->get_items(); 
    foreach ( $items as $item ) {
        $product_id = $item['product_id'];
        $itemdata = $item->get_data();
        $item_id = $itemdata['id'];
    }
    
    $gift_wrap = wc_get_order_item_meta(    $item_id, 'anbieter',true);
    $vendor_data_new = get_page_by_title($gift_wrap, '', 'wpsl_stores');
    $addresscustom = get_post_meta( $vendor_data_new->ID, 'wpsl_address', true );

    $response->data['line_items'][0]['store_address'] = $addresscustom;
    return $response;
} 
add_filter( "woocommerce_rest_prepare_shop_order_object","get_address_store", 10, 3 );

Code 2 (working):

function get_product_order_image( $response, $object, $request ) {
    if( empty( $response->data ) )
        return $response;
    
    $order_pid= $response->data['line_items'][0]['product_id'];
    $l_w_product_meta = get_post_meta($order_pid);
    $order_imgUrl= wp_get_attachment_url( $l_w_product_meta['_thumbnail_id'][0], 'full' );

    $response->data['line_items'][0]['cover_image'] = $order_imgUrl;
    return $response;
} 
add_filter( "woocommerce_rest_prepare_shop_order_object","get_product_order_image", 10, 3 );

Thanks for taking a time to read it :)